UI: Add new theme, update theme capabilities

I have always felt that the out of the box themes for OBS were quite
lacking, and have spent a lot of time going through and sorting out the
difficulties with the current setup. I've added a new themeID parameter to
several elements that were otherwise impossible to target with QSS in a
theme. Since Qt has pushed for the use of QML over QSS at this point,
these should be considered workarounds. Included is the theme I was
working on that can serve as a base. I'm hoping to encourage others to
make their own themes, so we can grow the available themes for OBS.

I am happy for any feedback on the theme itself, or other updates that
can me made to make creating new themes easier overall.

UI/window-basic-main.cpp

@@ -1213,6 +1213,7 @@ void OBSBasic::ResetOutputs()
 					this,
 					&OBSBasic::ReplayBufferClicked);
 
+			replayBufferButton->setProperty("themeID", "replayBufferButton");
 			ui->buttonsVLayout->insertWidget(2, replayBufferButton);
 		}