Find in SLiMgui should flash/highlight the found selection

[bhaller]

Find in SLiMgui should flash/highlight the found selection. This would make it easier to find visually. This is standard behavior on macOS, but the Qt folks don't plan to support it. They have a suggested implementation at https://bugreports.qt.io/browse/QTBUG-83147 that ought to be fairly straightforward. The only tricky part will be making sure that (a) the flash/highlight happens in all cases where it should, and (b) the flash/highlight gets reset back to the standard highlight color in all cases where it should.