Prevent division by zero when tab bar width is 0

[Adanos020]

Fixes #190

Basically, in DockArea::tab_bar_scroll, the scrolling coefficient is determined by a calculation which involves division by the available width. The problem is that when the available width is 0, the division results in infinity which is later multiplied by 0, and that produces a NaN here:

*scroll -= scroll_bar_handle_response.drag_delta().x * points_to_scroll_coefficient;

The scroll coefficient is later used all over the tab bar code, which spreads the NaN like a disease.

My fix for this is basically just turning off all tab bar code when the width is 0.

I also did a bit of clean-up: the Response returned from DockArea::tab_bar is only ever used to obtain the occupied Rect, so I made the function just return the Rect instead.