Remove DialogManager.

Replace remaining legitimate usages with [Keyed]DialogProviders.

Remove the DialogManager itself, all the getters for it, all the places keeping
references around to it, all the dialogs taking it in the constructor, and all
the partridges in pear trees that had one hidden away.

Change-Id: I0a1234960a4e08c0a7c6afc08a5432102b17c125
Reviewed-on: http://gerrit.dmdirc.com/3048
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>