Patch for #86 AND #149

This adds native styling for menus, however it will break some skins, due to the removal of the global * { -moz-appearance: none }. However this is pretty easy to fix as skin author. Do not merge this until it's confirmed to be working on Linux (DE shouldn't matter as this uses GTK styles), Mac OS and Windows.
Please check at least the main menu styles (including submenus, checkmenuitems, radiomenuitems, hoverstates, context menus, dropdown menus [search, mediaveiws, panes etc.]).