UI : Counter view improvements

[tdurand]

• Very hard to get for the user that you have to double click to finish setting up. Make it more verbose that one is currently in edit mode.
• Counter text (number) of polygon should be centered
• While in setting up polygon counter mode, I got sometimes some blue glitches (see screenshot)
• Counter details stats: can't be read if the area of interest is close to the upper side of the screen