Simplify extension version check

Quoting the main commit:

When adapting the check to the new versioning check, we just blindly
copied the old behavior for stable/unstable versions:

 - stable releases must have matching major numbers
 - unstable releases must match major and minor ("alpha", "beta", "rc")

That worked for the old even/odd scheme, but now has the absurd effect
that we consider an extension that lists "40.alpha" in its shell-version
incompatible with "40.beta", but compatible with "40.2".

At least this provides us with a good opportunity to reconsider the
behavior. While it is true that breakage is much more likely between
unstable releases, in practice extensions are either following shell
development closely or update once around the time of a stable release.

For the former, the stricter check isn't usually too useful (as the
extension releases around the same time as gnome-shell anyway).

For the latter, it's annoying that ".rc" is treated differently from
".0" and requires an update to become compatible.

The latter is also by far the more common case, so update the check
to only match on the major version regardless of whether a release
is stable or unstable.

Fixes #3787 (closed)