Also add some debugging output in the "positioning" domain.

Internal function for debugging support. It supports debug
domain filtering through the PANEL_DEBUG environment variable
(PANEL_DEBUG=1 for all debug output and for example
PANEL_DEBUG=positining:struts for only the positioning and struts
debug messages.

For now this is always enabled (ie. compiled) so everyone can easily
give me debug output without recompiling.

This function was designed to check if the plugin
runs in kiosk mode, but as an alternative there is now
xfce_panel_plugin_get_locked() and plugins should
implement this on their own (I even think nobody
used the function).

The old xfce_allow_panel_customization() is now a
macro that defaults to FALSE.

Each panel can be locked individually though Xfconf and
new API function xfce_panel_plugin_get_locked() for plugins;
although it might be safer for plugins to directly check the
Xfconf channel.

When a launcher plugin is removed, it tries to cleanup
the desktop dir. Hower this might fail because of leftover
files in the (re-used) directory from a previously
craches launcher.

Only print this in the terminal, because nothing is going
'wrong', just not so nice.

Add a quick signal back to the code and improve the internal
handling so the objects are better isolated.

Bug #5707.

With this function a plugin can remove itself from the panel
without user intervention.

Same implementation as old panel, but this time in
a loop to drop a lot of duplicated code.

Some styles properties for the minimized icon lucency,
max width chars and ellipsize mode, that were previously
hard-coded in the source.