Add icons for frame buttons

Add icons for frame buttons

Add window-minimize, window-maximize, window-restore and window-delete
icons to the builtin icon theme. These will be used for icons in
the window buttons, and the expectation is that the icon theme
will provide icons matching the desired decoration style.

window-delete is used instead of window-close, since window-close
is also used for GTK_STOCK_CLOSE, and the two may require different
styles when used inside the application vs in the window frame.