Preferences for choosing the icon set to use. It works on the fly, but

Preferences for choosing the icon set to use. It works on the fly, but
the size of icons may be wrong when switching between sets with different
normal icon size. One needs to save the preferences, restart LyX and choose
"Normal-sized icons" from the toolbar menu to have the real normal size.
A minor glitch is that the name of the icon sets is translatable, but
currently only the translation for "Default" works. The problem is that I
don't know how to ask gettext for an inverse translation when saving the
preferences. Another possibility would be hardcoding the order of the names
in the combo box (such that to not rely on their names), but that is ugly
and prone to errors if/when other sets are added.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@38041 a592a061-630c-0410-9148-cb99ea01b6c8