[Companion] Inputs and Mixes lists formatting & DnD fixes: (#4919)
* Fix/improve margin/padding issues, consistent between inp/mix views, better x-platform and older Qt (closes #4599, closes #4341);
* Allow dropping onto lines instead of only between them;
* Fix dropped data always going to line above the actual drop target (closes #4622);
* Always force DnD move operations between windows to copy instead (fixes deletion of any selected line in target window, and source line was never actually removed but looked like it was);
* Add alternate-row coloring for each Input/Mixer group;
* Fix non-contrasting selected row font color on Linux & OS X (was black on blue);
* Add actions to change font size (size persists until app close);
* Show inactive lines with dimmer font for input/channel name;
* Make drop indicator more obvious;
* Restore mouse hover highlight (Win), which also helps indicate actual drop target (also re: #4622);
* cosmetics: Code indentation in MixersListWidget.