(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.