Add StringUtils class + tests.

These are from the TextPane, but seem fairly general purpose.

Change-Id: Ic119eb461a365b4f7050715c6bc0abd5c034e57c
Reviewed-on: http://gerrit.dmdirc.com/4081
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager