Pass the current icon theme directory to plug-ins through the
config message, and add a gimp_icon_theme_dir() libgimp function
for retrieving it.  Note that we already have a similar
gimp_icon_get_theme_dir() PDB function, which we keep around, since
it can be used to dynamically query for the current icon dir,
unlike the former, and since it returns a dynamically-allocated
string, while the rest of the config-related functions return
statically allocated strings.

Use the new function, instead of gimp_get_icon_theme_dir(), in
gimp_ui_init().  This allows gimp_ui_init() to run without making
any PDB calls.  Consequently, this allows us to start plug-ins that
call gimp_ui_init() without entering the main loop in the main app.
We're going to add a plug-in that displays an interactive dialog
while the main app is blocking waiting for an operation to
complete, and we need to be able to start the plug-in without
entering the main loop, to avoid the possibility of arbitrary code
being executed during the wait.

Bump the protocol version.