Replace TextPaneBoundedRangeModel with the default.

Seems functionally identical aside from a couple of bugs in the
TextPane one :-)

Change-Id: I2a28e7e7761224bab435b323a83c6586bda71067
Reviewed-on: http://gerrit.dmdirc.com/4084
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager