Gnc-Prefs: migrate search dialog and saved window geometry preferences

Note: the saved window geometry options (position and size) are
converted into a type that is incompatible with gconf. This has two
consequences:
1. while testing the gconf backend is used by gsettings. The
incompatible option (last_geometry) won't be saved in gconf. You will
see an error because of this. It does work when switching to gsettings'
native backend

2. The old geometry data won't be converted at user migration time (ie
when the user starts gnucash-with-gsettings for the first time. I don't
think this is very dramatical. The next time a dialog is used, the last
saved position is saved again already automatically.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23229 57a11ea4-9604-0410-9ed3-97b8803252fd