GSettings: only load backend when installed

The preferences backend can't work from the source environment because
it's schemas won't be compiled. This situation is only relevant for testing,
which is done from the source (or build) tree.
Note that a few preferences are needed by the xml backend. If no preferences
backend is loaded, some sane defaults will be used instead. Again this should
only matter for testing, in which case the real preferences are not relevant
anyway.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23473 57a11ea4-9604-0410-9ed3-97b8803252fd
Fix python test

It no longer requires access to the installed gsettings schemas.
The default value for XDG data dirs didn't work anyway because
it used a ; instead of a : as separator (as it should for the
environment file).

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23474 57a11ea4-9604-0410-9ed3-97b8803252fd