Master issue for missing features in uiArea

Merging a bunch of issues here.

- [ ] Support for trackpad gestures (see #81 for details)
- [ ] Independent control of uIArea sizing in both horizontal and vertical directions (to allow areas to be scrollable in only one direction, for instance) (see #82 and linked issues for details)
- [ ] uiAreaSetMinSize() to determine the minimum allowable size of a uiArea for the minimum size of container size allocations; will also need to be axis-dependent
- [ ] uiAreaQueueRedraw() for only a specific part of a uiArea
- [ ] Un-expose unconditional redraw of a uiArea on window resize; fine-tune redrawing after resize as needed (see #251 for details)
- [ ] Related to the previous one: properly implement clipping on Windows (TODO get relevant stackoverflow questions I asked on the matter)
- [ ] Allow keyboard event handlers to know what the key on the keyboard is from the user's perspective (so on a dvorak keyboard, when pressing U, `Key` will still be `'f'` but this new field would be `'u'`) — **but this must be clearly marked as only for the purposes of user interaction, NOT FOR TEXT INPUT** (and I have to remember a good reason why; TODO)
- [ ] Having controls overlaid on top of uiAreas; will need to decide how many can be done and figure out how they could or should interact with scrolling and sizing