browser-plugin: Fix warning when running in Chromium

Work around Chromium setting GDK_NATIVE_WINDOWS. It's not used
for GTK+ 3.x, so unset it.

https://bugzilla.gnome.org/show_bug.cgi?id=693092
browser-plugin: Fix running under recent gnome-terminal

--disable-factory isn't supported any more.
browser-plugin: Fix running gdb when it is a script

Pass a full path to launch gdb to avoid running a helper script.
browser-plugin: Fix warning for hidden plugin

Fix warning on startup when we're not being embedded, we
can't realise an unanchored window.

https://bugzilla.gnome.org/show_bug.cgi?id=693092
browser-plugin: Fix warning inhibiting idle for hidden plugin

When the plugin is hidden, we shouldn't be inhibiting idle to
prevent the screensaver coming on. It also fixes warnings
at run-time as gtk_application_inhibit() expects a real window,
not NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=693092