Click-drag on view boundary in overview to resize visible view area

Follow on from JAL-2611. The corners and sides of the visible view box in the overview should be draggable, enabling the user to quickly rescale the view (i.e. have font and row/column geometry recomputed) to encompass a particular area of the alignment.

Ideally, mousing over the visible view box should offer appropriate visual cues (horizonal, vertical or diagonal double ended arrow), and a 'preserve aspect ratio' mode should also be provided where the view is scaled uniformly with respect to it's current visible row/column ratio.