Remove all UpdateChecker.getManager() calls.

Down with static methods! Temporarily up with deprecated methods on horrible
global state objects!

Change-Id: Ia279f9199e8ca40dc3031234569a44da042fc617
Reviewed-on: http://gerrit.dmdirc.com/3027
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager