Fix edge scrolling

**Issue by [zeyangl](https://github.com/zeyangl)**
_Friday Nov 25, 2016 at 02:19 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/issues/142_

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Civ 6 map scrolling suffers from a few issues.
1. top "status bar" blocks scrolling. I have to put the cursor slightly below the status bar.
2. bottom right UI blocks scrolling as well.
3. left and right scrolling trigger overlaps with the edge buttons.
4. no sensitivity settings.

I'm putting this up as a feature request. It would be nice if this can be properly implemented.

No files, tests, or entry points are named. Start by locating the map-scrolling and edge-button UI handlers, then trace how the top status bar and bottom-right UI affect their trigger areas. Done means scrolling works at each map edge without those UI elements blocking it, with the requested sensitivity behavior defined and covered by the issue's four reported cases.

Written by the indexing model from the issue text.