Create gsettings schema entries for all gconf keys currently in use

These entries were generated using a conversion tool supplied with
gsettings:
gsettings-schema-convert --force --gconf --xml -u
                         --schema-id <GSETTINGS_SCHEMA_ID>
                         -d GETTEXT_PACKAGE ../schemas/<GCONF_SCHEMA_FILE>
                         -o <GSETTINGS_SCHEMA_ID>.gschema.xml.in

Missing keys (gconf didn't *require* schema keys) were manually
added.

The only keys not converted are those that are used multiple times
in different paths, like budget/account register column widths.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23224 57a11ea4-9604-0410-9ed3-97b8803252fd
Amend XDG_DATA_DIRS if not installed in default prefix

This ensures gsettings schemas are properly found

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23225 57a11ea4-9604-0410-9ed3-97b8803252fd