Allow resizing window and making it non-floating

[maximbaz]

Right now it's impossible to resize the window, to make it fullscreen or non-floating (if you do, you can't draw inside, for example).

When resizing a window, the image inside should be scaled appropriately.

This would have been a workaround for #56, but I'm also thinking that sometimes it could be convenient to just open swappy window in full screen for comfortable editing.

[jtheoof]

Fair request @maximbaz

[heroin-moose]

Bump?