Move swap/cache and temporary files out the GIMP user config dir:

libgimpbase: add gimp_cache_directory() and gimp_temp_directory()
which return the new default values inside XDG_CACHE_HOME and the
system temp directory. Like all directories from gimpenv.[ch] the
values can be overridden by environment variables. Improve API docs
for all functions returning directories.

Add new config file substitutions ${gimp_cache_dir} and
${gimp_temp_dir}.

Document all the new stuff in the gimp and gimprc manpages.

app: default "swap-path" and "temp-path" to the new config file
substitutions. On startup and config changes, make sure that the swap
and temp directories actually exist.

In the preferences dialog, add reset buttons to all file path pages.