libpanel-applet: add panel-applet-private.h...

... and move private functions from factory to this file. Later
we will have at least one more private function.
libpanel-applet: add process type to private data

We will need to know this as soon we will create GtkPlug and
GtkSocket only for out of process applets.
libpanel-applet: use g_define_type_with_private
libpanel-applet: delay GtkPlug creation

We are going to create GtkPlug only for out of process applets and
constructor is first place where we will know applet process type.
libpanel-applet: store factories in hash table
libpanel-applet: store applets in hash table
libpanel-applet: return process type and applet uid

Return applet process type and generated uid from factory. Process
type will be needed to know if we should create GtkSocket, but uid
will be used to get applet widget.
libpanel-applet: reformat panel-applet-factory.h
libpanel-applet: add panel_applet_factory_get_applet_widget

This function will be used to get applet widget from factory.
libpanel-applet: reformat panel-applet-private.h
make it possible to get applet widget

We will use this to get applet widget from factory. This will
allow to bypass GtkPlug/GtkSocket.
don't use GtkPlug/GtkSocket for in process applets

1. This will allow panel to exit/stop without crashing when using
in process applets. It was crashing because of following assert:
g_assert (widget->priv->verifying_invariants_count > 0);

2. In process applet will not have extra background. Transparency
should now work as expected. This is still problem with out of
process applets.
theme: remove background color from menu bars

Setting background color for menu bars is not needed. Also it
breaks transparent panels.
build our applets as in process applets

1. If applets are build as out of process applets then they are
ignoring styling added in gnome-panel/gnome-panel.css.

2. Transparent panels will work better with in process applets.

3. Debian and Ubuntu does this by default.
doc: add out-of-process