proper resolution selection

[nczempin]

trafficstars

in issue #54 I added the option to switch between 800x600 and 1280x720; nothing else.

It should really be possible to choose appropriate resolutions: a) for windowed mode pretty much any values of width and height are possible (up to the maximum) b) for fullscreen we need to ask the graphics card what resolutions it supports.

a) would require some kind of number picker and/or text field, b) needs a combo box (pop up list).

Either of these would need to be added as GUI widgets before we could support the respective functionalities.

b) would probably have a higher priority because it would work for both fullscreen and non-fullscreen, whereas a) only works in windowed mode.