Updates font management for hidpi

Improvements:
- Cleanup of default options files to use OS font by default
- Comments updates
- Simplify font size management as Pango/Gtk directly manage pt/px conversions

Bug fix:
- If Preference panel is closed by "cancel", font size was not reinitialized to previously saved one
- Font size was specified in "RawTherapee.css" theme and so user choice could be ignored

rtdata/themes/RawTherapee.css

@@ -26,7 +26,6 @@
 * {
     color: #BBBBBB;
     text-shadow: none; /* Keep at none, or suffer serious performance issues */
-    font-size: 1em;
 }
 
 *:disabled {