[GTK] Add webkit_web_context_set_disk_cache_directory to WebKit2 GTK+ API
https://bugs.webkit.org/show_bug.cgi?id=111848

Reviewed by Anders Carlsson.

This allow applications to set a custom directory for the disk
cache.

* UIProcess/API/gtk/WebKitWebContext.cpp:
(webkit_web_context_set_disk_cache_directory):
* UIProcess/API/gtk/WebKitWebContext.h:
* UIProcess/API/gtk/docs/webkit2gtk-sections.txt: Add new symbol.
* UIProcess/API/gtk/tests/TestMain.cpp:
(removeNonEmptyDirectory):
(main): Use a different temporary disk cache directory for every
test.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148887 268f45cc-cd09-0410-ab3c-d52691b4dbfc