Added 4 new D-Bus API methods: show, hide, get_tab_name, get_tab_count

I added four new D-Bus API methods for consumption in a GNOME 3 extension I have been working on ([gnome-tabs-indicator](http://github.com/pztrick/guake-tabs-indicator/tree/dbus_extras)). They are pretty minor and don't impact any core Guake functionality.

* `org.guake.RemoteControl.show` : displays the terminal overlay
* `org.guake.RemoteControl.hide` : hides the terminal overlay
* `org.guake.RemoteControl.get_tab_name + index (int)`  : returns tab name
* `org.guake.RemoteControl.get_tab_count` : returns number of tabs (terminals)

The extension integrates Guake terminal tabs into the GNOME Shell chrome for a clean look in GNOME 3. (There is a screenshot in the [README.md](http://github.com/pztrick/guake-tabs-indicator/tree/dbus_extras)).

_(The existing `show_hide` method, as a toggle, wasn't appropriate for my use case.)_