Handle multiple desktop files with different StartupWMClass

Some system installed apps define common/default StartupWMClass in
system-installed .desktop file.

Application then create extra .desktop files in user's local
applications directory with additional (more specific) StartupWMClass
entry.

This patch will match specific window class (WM_CLASS instance name)
before matching common window class (WM_CLASS class name).

See: https://www.x.org/releases/X11R7.7/doc/xorg-docs/icccm/icccm.html#WM_CLASS_Property

BUG: 358277