vera-session: exit after a maximum number of restart, to avoid infinite loops. Also ensure we start vera with --disable-autostart in case of a crash.

We do not want to restart possibly autostarted applications two (three? four? let's hope one!) times.
Moved the ExitHandler to dbus/, and rename it using a more generic name.

As we do not want to pollute too much the Session Bus, we are integrating the ExitHandler with the future vera dbus service.
PluginManager: support plugin unloading. Also implemented new methods to load a plugin singularly.

Methods added:
get_plugin_info()
load_plugin()
load_plugin_from_plugin_info()
unload_plugin()
unload_all_plugins()
DBusService: export methods to dynamically load and unload plugins.

Added methods:
UnloadPlugin()
LoadPlugin()
PluginInterface: API change -- added shutdown() method, that will be called when the plugin has been unloaded.
DbusService and PluginManager: ensure we invoke startup() on the loaded plugin.

A new SessionPhase has been added, SESSION. This will be used for dynamically
loaded plugins.

Added methods:
(PluginManager) startup_plugin_from_name()
Catch SIGQUIT, SIGTERM and SIGINT, and use them to end the main loop, because after it vera now will do some cleanup.

Implemented methods:
vera: quit()
DBusService: quit()
Screenshot: quit()
Updated gitignore.
Merge branch 'master' into debian
0.24-1