extensionSystem: Fix reloading on version-validation changes

The current code only works for enabled extensions, which means
that extensions that were marked OUT_OF_DATE cannot be enabled
without a restart when disabling the version check.
Fix this by reloading all extensions while making sure to only
enable any extensions when we're supposed to.

https://bugzilla.gnome.org/show_bug.cgi?id=736185
extensionPrefs: Remove version check from extensionAvailable()

It is confusing to treat outdated extensions as if they didn't exist
in some places, but like any other extensions elsewhere. In particular
gnome-tweak-tool still allows to launch prefs for them while making
it clear to users that the extension will not work - opening the
list of installed extensions in that case is unexpected and confusing.
Just remove the version check for now, we will soon follow tweak-tool
and use it to disable the extension switch instead.

https://bugzilla.gnome.org/show_bug.cgi?id=736185
extensionPrefs: Improve handling of OutOfDate extensions

Our current UI is horrible in dealing with outdated extensions - we
don't give users any indication at all that something is not working as
expected (unless hiding the prefs button as we did until the last commit
counts).
To fix this, make the enable switch insensitive to indicate OutOfDate
extensions.

https://bugzilla.gnome.org/show_bug.cgi?id=736185