Added the functionality to specify the StartupWMClass in mLauncher

[Manuel-Kehl]

I mostly use mLauncher to create "Chromium webapps", i.e. chromium --app="URL" as executable. While assigning icons to the .desktop file works for launching the app, it does not work at runtime, as it returns to the default Chromium icon. To fix that, the URL (without http[s]://) has to be added as StartupWMClass, which is now possible via mLauncher :wink:

:bulb: For the future one could even think about creating a single button "create Chrome/Chromium webapp" with a popup that handles URL processing, so that the user simply specifies the URL, name and icon. What do you think about that?