Unify and simplify how we determine the default browser
Currently we have three sources of truth for the default browser:
- The BrowserApplication entry in kdeglobals
- The default app for text/html
- The default app for x-scheme-handler/http
All are set by the default apps KCM, but e.g. setting Firefox as the default browser from itself only changes the scheme handlers and not kdeglobals
There's not really a reason to have it set in kdeglobals so always check only the x-scheme-handler/http app