Add display properties to DisplayableEvents.

This will allow listeners to change how events are displayed,
for example setting the default FG colour to red when the
user is highlighted.

Depends-On: I21f9babe74a90b5cfea6daba815c08e7a4b5afa5
Change-Id: I0ac716ee335ed40531af698fe475d077aba2766d
Reviewed-on: http://gerrit.dmdirc.com/3833
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Update submodules: plugins (new commits)

plugins:
    67cf8a0 Chris Smith <chris@dmdirc.com>
            Support for display properties.

            Depends-On: I0ac716ee335ed40531af698fe475d077aba2766d
            Gerrit: http://gerrit.dmdirc.com/3834

Change-Id: I3da36263b67f064463ffa5f00e0d57d67288ab9f