Apps: Add mpv, update and refresh others

A few redone to match newer elementary style, palette.
Should look a little better in dark themes with updated borders.
Improve some consistency between sizes.

New
- mpv

Updated/Refreshed
- menulibre
- multimedia-player (device icon, but used for some apps)
- preferences-desktop-font (based on upstream icon, replaced PNGs with SVGs)
- postscript/pdf viewer
- virtualbox