Use StartupWMClass to associate window and applications

Some applications (such as most Java apps, as well as Chrome Web apps) ship
with desktop files that have the wrong name, but whose StartupWMClass
field contains the right value. Therefore first check that key, against
both the class and instance part of WM_CLASS, and only use the filename
if nothing else works.

https://bugzilla.gnome.org/show_bug.cgi?id=673657