Remove AC_USE_SYSTEM_EXTENSIONS, seems it confuses recent automake/autoconf which sets a wrong path to install.sh

No regression with automake 1.11 / autoconf 2.63.
Appease autohell wrath^Wwarnings about putting m4 stuff in a m4 subdir.
Declare variable at the beginning of the function
Remove g_thread_init() calls, unneeded those days
Fix keyboard navigation on the actions radio buttons (bug #11325)

Use a gtk_table instead of a vbox to fix keyboard up-down navigation between actions.
Before that, the open with combobox radiobutton was last in the navigation, while in
the middle on the screen.
Merge branch 'master' of git://git.xfce.org/apps/xfce4-screenshooter