Convert PluginFilesHelper away from ResourceManager.

Initial testing seems to work.  Will add unit tests later.

Change-Id: I3824ce74f41e70be605004fa4b0973be837f73e3
Reviewed-on: http://gerrit.dmdirc.com/4033
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>