GSettings: add functions to listen for changes

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23218 57a11ea4-9604-0410-9ed3-97b8803252fd
GSettings: add functions to get/set most common data types

These are
- integers
- floating point numbers
- strings
- enums
- arbitary combinations of values in a GVariant (this will
  be used to replace the current GConf list getters/setters)

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23219 57a11ea4-9604-0410-9ed3-97b8803252fd