[single window mode] Don't resize window if last chat tab is closed / 'Always show tabs' option
If last chat tab is closed Gajim resizes window to roster size. In tiling wm it causes that roster is resized and after reopening chat tab it occupies whole window and user has to resize roster again and again.
The would be a blank space in place of chat tabs if none is opened which would prevent window resizing which doesn't work as expected in all environments.