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

Closes jp9000/obs-studio#1141

Closes jp9000/obs-studio#1141

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)

QToolTip's (background) color(s) would not apply on windows, work-around
taken from https://forum.qt.io/topic/24023/

When an item is selected but not focused, it would be too similar to the
non-selected color.

This fixes an issue where the borders for certain types of windows would
not match the intended border style/color.  It was supposed to be 1
through 6 for frameShape, but I ended up putting 5 twice.

Fixes an issue where the border color/style would not be consistent
across different operating systems

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