Use services for frame managers.

To add a frame manager, a plugin must export a FrameManagerProvider instance.
This will drop users back to using a treeview as their previous config setting
will be wrong (it needs to point to the Provider class now), and I don't think
it's worth adding migration code given we know of 0 users of the buttonbar.

This also enables us to dependency inject the frame managers.

Change-Id: Ic66997625587e1adc0ee6ed1c7e38fba31ec47e7
Reviewed-on: http://gerrit.dmdirc.com/3283
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>