Name upgrade functions after full version
Ensure each upgrade function is run only once
Move variable near where it is used
Define _s once in parent function
config.setting -> _s in upgrade_to_v1_0_0_final_0() too
Add FIXME about broken upgrade hook
Make ConfigUpgradeError message more useful including traceback
version_to_string(): be smarter and accept list with strings
Simplify hook registration by detecting upgrade hook functions
Add a comment about adding an upgrade hook
Add missing % character escaping
Setup gettext before _upgrade_config() call
Move upgrade message boxes to _upgrade_config() instead of MainWindow

Re-format messages so they wrap nicely.

Remove related FIXME
Move upgrade code to its own file, no need to be a Tagger method
Log explicit messages for config upgrades and drop now useless comments

Use log.info() instead of log.debug() as it is useful for any user to know
config options were modified.
Merge pull request #202 from zas/upgradepath2

Improve and fix upgrade hooks code