Crash recovery for plasma-shell

We restart plasma-shell after crashes. When it crashes subsequently on startup,
and more than two times in a row, we give up in order to not endlessly loop.
Once the shell process stays alive for at least 15 seconds, we reset the crash
counter, so a later crash, at runtime rather than on startup will still be
recovered from.

This logic is very similar as to how kwin handles it.

DIGEST: