Add GSettings schemas and build rules

These are converted from their GConf equivalents.
Some caveats:
- During the code updates from GConf to GSettings both systems
  use the same backend and hence need the same key and
  path names or they won't see each others changes.
- For this reason the conversion in this commit
  preserves the GConf key names even though these can
  contain illegal characters for GSettings.
- Once all code is successfully converted
  to GSettings and we have a preferences conversion
  script in place, we need one more round of fixing
  the keys and paths (/apps is deprecated in GSettings
  in favour of /org)

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23216 57a11ea4-9604-0410-9ed3-97b8803252fd
Add some initialization code for GSettings

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23217 57a11ea4-9604-0410-9ed3-97b8803252fd