Check for working gconf and add fallback trigger key (closes bgo#666136)

Meld presently just checks whether gconf is importable before using it,
which misses some cases where gconf can't talk to the bus or isn't
running.

This patch actually checks for functioning gconf by trying to write a
key, and also adds a file that, if present, forces the ini-file
fallback. This second feature is to avoid switching between
configuration backends in situations where gconf sometimes works and
sometimes fails.
Refactor the gconf testing slightly