[EXPLORER]: Fix most problems with the size of the taskbar. (Most problems when run...

[EXPLORER]: Fix most problems with the size of the taskbar. (Most problems when run in windows. Running on ros still isn't perfect.)
- CTaskSwitchWnd: Use the height of the start button as minimum height in horizontal mode.
- CTrayClockWnd: Actually process WM_THEMECHANGED message.
- CTaskSwitchWnd: Use SM_CYSIZE as the height of the buttons of the toolbar.
- CStartButton: Simplify calculating the minimum size of the button.
- CTrayWindow: Don't use IsThemeActive. Checking if OpenThemeData returned non null is enough.

svn path=/trunk/; revision=74231