Set WM_CLASS in Linux

[th-we]

Prevent that you get multiple icons for StopWatch in the "launcher" on Linux once an instance of StopWatch is running. (There'll be one launcher icon and one "generic" icon for the running instance - see screenshot.) For this, X's WM_CLASS property has to be set.

This should be done on top of PR #117. I don't have time to do this right now, but plan to do it later (but I'm not angry should anyone else do It ;-)). Some example code that might be useful:

• https://bitbucket.org/hindlemail/settingwmclass/
• https://github.com/sillsdev/libpalaso/blob/master/SIL.Windows.Forms.GeckoBrowserAdapter/NativeX11Methods.cs