Status of launched processes should be trackable

[g7]

We do not have any way to determine when a Launcher()-started application has been fully initialized.

This creates some bugs in the current implementation because we launch core applications (like the WM and the panel) almost concurrently and, for example, the panel is rendered on screen before the WM is fully initialized and it will not stick on every desktop because when trying to set the relevant property the WM was not ready.

Another (pretty major) issue we faced is that some tray icons may render before the panel's initialization, thus they will never get displayed on the status bar.

This of course is more evident on slower systems and (for obvious reasons) in the live environment.

A way to solve this is to use Startup Notifications but they are a PITA to implement and pretty X dependant.
Thankfully GLib comes to the rescue with its AppLaunchContext, which may be a great and somewhat easy way to have startup notifications in vera.

The goal of this bug report is to track the progress on this issue.

Eugenio