This changes all of the icons from png to svg. With svgs, scaling is
unlimited. With the svgs, the issue of the @ symbols with the Windows
updater is no longer an issue.

I changed the colors of the icons to a light gray (#d2d2d2), in the
dark themes, because I thought they looked better with this color.

The mute, unmute, plus, minus, up, down and expand icons are from the
Feather Icons set. https://feathericons.com/

The visibility icon is from the Octicons set. https://octicons.github.com/

The locked and unlocked icons are from the Open Iconic set.
https://github.com/iconic/open-iconic

- Fixes Q_PROPERTY macro syntax
- Removes excessive notification declaration and slot
- Removes unneeded dynamic property
- Improves Themes versatility

The system theme was named Default even though the default theme is Dark.
This addresses that by renaming Default.qss to System.qss. I've made it
backwards compatible so users already using this theme are not affected.

The theme list now shows up as:
-System
-Dark (Default)
-Acri
-Rachni

I have also made it so that you can specify the default theme in the
UI config file.

This changes the bottom area color for the about dialog
for the light theme. This color makes it more consistent
for the theme.

The decay rate of the audio meters can now be selected in the audio
settings.  The values are:

- "Fast"   (OBS default, 40 dB / 1.7s)
- "Medium" (Type I PPM,  20 dB / 1.7s)
- "Slow"   (Type II PPM, 24 dB / 2.8s)

Closes jp9000/obs-studio#1143

The following features have been added to the audio-meters:
 * Stereo PPM-level meter, with 40 dB/1.7s decay rate.
 * Stereo VU-level meter, with 300 ms integration time.
 * Stereo Peak-hold meter, with 20 second sustain.
 * Input peak level color-squares in front of every meter.
 * Minor-ticks for each dB.
 * Major-ticks for every 5 dB.
 * Meter is divided in sections at -20 dB and -9 dB.

The ballistic parameters chosen here where taken from:
 * https://en.wikipedia.org/wiki/Peak_programme_meter (SMPTE RP.0155)
 * https://en.wikipedia.org/wiki/VU_meter

In the rework I have removed any ballistic calculations from
libobs/obs-audio-controls.c making the calculations here a lot more
simple doing only MAX and RMS calculations for only the samples in
the current update. The actual ballistics are now done by just
the UI/volume-control.cpp because ballistics need to be updated
based on the repaint-rate of the user-interface.

The dB to pixel conversion has been moved from
libobs/obs-audio-controls.c to UI/volume-control.cpp as well to reduce
coupling between these two objects, especially when implementing the
major- and minor-ticks and the sections.

All colors and ballistic parameters are adjustable via QT style sheets.
There are slight differences in colors for each of the themes.

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

Certain output formats don't support multiple tracks, so it's important
to warn the user if they select multiple tracks.

Also warn the user if they select no tracks.

OBSHotkeyEdit is similar to QKeySequenceEdit, except less terribad on OSX, while
OBSHotkeyWidget provides a container that automagically encapsulates multiple
bindings per hotkey

OBSHotkeyLabel in combination with OBSHotkeyWidget provides mechanisms for
dealing with hotkey pairs (see upcoming usage in window-basic-settings.cpp)

Links up certain controls via a "themeID" property so the icons can by
dynamically changed via the qss file if needed.