Rather than using a custom window migrate to custom items in a
GtkMenu. To that end, change from a GtkTreeView to GList internally.

Also put more trace/dbg messages in place so it's easy to track
these issues when upower breaks itself again in the future.

Dynamically changing the icon in the panel itself looks horrible.
Revert and improve it later.

This patch moves some code around, mostly putting shared stuff into
power-common inside common/. It also creates a new plugin to monitor
all the batteries in one icon on the panel rather than multiple icons
inside the system tray. It also removes the power-info application
as that will be moved inside the newly reworked settings dialog.