This is to prevent confusion both when prefixing commits and when
reading the directory structure for the first time.

(Non-compiling commit: windowless-context branch)

This makes it so that OBSQTDisplay now uses/controls an obs_display
object directly (rather than having the owner have to associate an
OBSDisplay with it).  It was separated before because the main window
for the basic UI would was using the "main preview" stuff before the
windowless context and had to be handled differently, so you couldn't
just associate an obs_display object with OBSQTDisplay, meaning that all
"secondary" previews such as properties/filters/etc had to handle the
obs_display alone, which caused a lot of needlessly duplicated code.

Also adds a DisplayCreated signal to allow owners to be able to add
callbacks and such on creation of the actual obs_display context.

Found by clang-3.6 via -Winconsistent-missing-override

This allows knowing when the graphics viewport itself changes in size
(in case the window with the graphics viewport does not change size)

 - I seem to have fixed ths issues with the main preview widget.  It
   seems you just need to set the right window attributes to stop it from
   breaking.  Though when opengl is enabled, there appears to be a weird
   background glitch in the Qt stuff -- I'm not entirely sure what's
   going on.  Bug in Qt?

   Also fixed the layout issues, and the widget now properly resizes and
   centers in to its parent widget.

 - Prevent the render loop from accessing data if the data isn't valid.
   Because obs->data is freed before the graphics stuff, it can cause
   the graphics to keep trying to query the obs->data.displays_mutex
   after it had already been destroyed.

```-----------------------------------------------
Notes and details
```

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Why was this done?  Because wxWidgets was just lacking in many areas.  I
know wxWidgets is designed to be used with native controls, and that's
great, but wxWidgets just is not a feature-complete toolkit for
multiplatform applications.  It lacks in dialog editors, its code is
archaic and outdated, and I just feel frustrated every time I try to do
things with it.

Qt on the other hand..  I had to actually try Qt to realize how much
better it was as a toolkit.  They've got everything from dialog editors,
to an IDE, a debugger, build tools, just everything, and it's all
top-notch and highly maintained.  The focus of the toolkit is
application development, and they spend their time trying to help
people do exactly that:  make programs.  Great support, great tools,
and because of that, great toolkit.  I just didn't want to alienate any
developers by being stubborn about native widgets.

There *are* some things that are rather lackluster about it and design
choices I disagree with though.  For example, I realize that to have an
easy to use toolkit you have to have some level of code generation.
However, in my personal and humble opinion, moc just feels like a
terrible way to approach the problem.  Even now I feel like there are a
variety of ways you could handle code generation and automatic
management of things like that.  I don't like the idea of circumventing
the language itself like that.  It feels like one giant massive hack.

--------------------------------------------------
Things that aren't working properly:
--------------------------------------------------
 - Settings dialog is not implemented.  The dialog is complete but the
   code to handle the dialog hasn't been constructed yet.

 - There is a problem with using Qt widgets as a device target on
   windows, with at least OpenGL: if I have the preview widget
   automatically resize itself, it seems to cause some sort of video
   card failure that I don't understand.

 - Because of the above, resizing the preview widget has been disabled
   until I can figure out what's going on, so it's currently only a
   32x32 area.

 - Direct3D doesn't seem to render correctly either, seems that the
   viewport is messed up or something.  I'm sort of confused about
   what's going on with it.

 - The new main window seems to be triggering more race conditions than
   the wxWidgets main window dialog did.  I'm not entirely sure what's
   going on here, but this may just be existing race conditions within
   libobs itself that I just never spotted before (even though I tend to
   be very thorough with race conditions any time I use variables
   cross-thread)