Try to sort all GIMP_ICON_* defines into FDO categories like in
https://specifications.freedesktop.org/icon-naming-spec/latest/ar01s04.html
Add defines for all icons we override, rename some icons to their FDO
standard names, and mark the ones we duplicate with a comment so we
don't forget to rename those to standard names in 3.0.
It's deprecated in GTK+ 3.x, so we get rid of some deprecation
warnings. Also, it has always only been a utility function that hides
what is really going on in the widget hierarchy.
Add gimp_prefs_box_set_page_scrollable() which does what it says, and
make the tallest pages of the prefs dialog scrollable. This method
allows the dialog's smaller pages to still enforce a minimum height
for the window.
Remove the parameters from gimp_prefs_box_add_page() and instead
hardcode them once in GimpPrefsBox. This way they can also easily be
made configurable in the future.
