Cleanup statusbar template code a bit

* Unhardcode "pos" and "style" statusbar messages which were only
enabled when GEANY_DEBUG is defined and make them real possible
format chars.

* Move needless global "statusbar_template" into UIPrefs structure
with the other UI preferences, removing (now) pointless ui_finalize()
function.

* Rename "add_statusbar_statistics" to "create_statusbar_statistics"
and make it return a gchar* instead of passing in a GString argument
to update. Fixes a one-time "leak" of the GString and makes the code a
little easier to follow.

* Move the default statusbar template string to the top of the file
and use it as the default for the various preferences so the user has
something to base their customizations off of. TODO: check that the
N_() translations stuff works OK.
Improve Statusbar Template documentation

Mainly listing the available placeholders instead of pointing the
poor user deep into Geany's code.