WindowMonitor should trace Shell_TrayWnd even if it's not visible

[dechamps]

While investigating dechamps/RudeWindowFixer#3 I came across a trace where WindowMonitor logged WindowGone for the taskbar window (Shell_TrayWnd). The window reappeared later with the same HWND which suggests it was not actually destroyed - it's more likely that IsWindowVisible() flipped.

This is problematic because that means we are blind to any window property changes that occurred during the time the window was hidden.

[dechamps]

Actually, as discussed in dechamps/RudeWindowFixer#3, it might be that the real problem is that WindowMonitor loses track of windows that change bands.