Add MeterDisplay UI widget

Signed-off-by: James Tasker <james_tasker1@hotmail.com>
Make base meter type fill rectangle with solidcolour
Catch case when image in ImageMeter is null

Signed-off-by: James Tasker <james_tasker1@hotmail.com>
Add button to create new ImageMeters

Signed-off-by: James Tasker <james_tasker1@hotmail.com>