FYI: 4.7 panel and plugins

[Nick Schermer]

To continue this, I've fixed some stuff in the panel:

Introduced the pre-init function again. This is only possible inside
the wrapper, so when an internal plugin is loaded the panel will check
if there is a pre-init function defined by the developer, if so, it
will force the plugin to run external (technically this will only
happen then a user changes the desktop files to run it internal).
Xfdesktop patch has been updated accordingly.

The g_thread_init() functions have been removed from both the panel
and wrapper, no noticable speed up, but anyway, everybody happy.

There is also a /force-all-external property to launch all plugins in
their own wrapper (ie. everything external, whatever the developer has
set).

Support for the old external *applications* will be though (different
communication, no support for rgba and more), so for now it's not high
priority more important stuff to be fixed first. I put it on the todo
for now.

Nick