[custom win scrollbars] Scrollbar thumb doesn't highlight when hovered over or dragged

[bchintx]

Found while reviewing Pull Request #6305- "Custom scrollbars for Windows based on win 8".

Steps to Reproduce:

1. using branch 'tom/win-scrollbars', launch Brackets
2. open a source file that extends beyond one screen -- ie. so that the vertical scrollbar appears. Notice the appearance of the scrollbar thumb.
3. move/hover the mouse cursor over the thumb.
4. click and drag the thumb up and down.

Observed:
On Windows, hovering the mouse over the vertical scrollbar no longer highlights (ie. darkens) the scrollbar thumb. Same thing when dragging the thumb.

Expected:
That the scrollbar thumb would highlight while the mouse cursor hovers over the thumb area and while dragging the thumb. The highlighted behavior occurs on normal Windows 8 scrollbars.

This issue does not occur on Mac. It does occur on both Windows 7 and 8.

[marcelgerber]

@bchintx See #6305 (comment) for a Win8-like hover effect.

[bchintx]

Fixed with latest changes in branch. Thanks!