more X11 file dialog tweaks

* file name extension filter
* remember selected entry
* separate file-are L+R margin
X11 file dialog:

* add check-boxes
* add places
* show/hide hidden files
* tweak color schema
* re-work hovering
further X11 File Dialog updates:

* proper API
* support filesystem 'places'
* various UI alignment tweaks
* add outline around lists, shaded rects
add "recently used" list (so far only w/GUI for X11)
clean up file open dialog & add recently-used for win&OSX
OSX: allow key repeat
add X11 double buffering