Update GTK+2 to version 2.24.23
Update themes and include them into master
Use external web browser for the manual if WebKit is unavailable

Cherry-picked from commit 4dc16825b2cad223c2856886a086bc908556ecb.
Sets the help-browser to the system's web browser in gimpguiconfig.c, so
we don't need to set it separately in the gimprc file.

WebKit support on OS X needs more investigation, thus we stick with the
system's web browser for now.
Hide the help browser selection listbox if no help browser is installed

If we have the internal help browser not installed (e.g. because of 
missing WebKit) it makes no sense to offer it as an ever-failing option
to the user. Thus hide it and use the external web browser by default
then. 

Based on a patch by Simone Karin Lehmann.
Use the online help  by default

Because the user manual is not shipped with GIMP by default, it's
pointless to expect it installed. Thus setting the online help as
default in gimprc.