Fixed charmap plugin to show all configured characters

The charmap plugin actually lists 251 items, only a matrix of 25x10 was shown.

I changed the code to calculate the height of the matrix, preventing the last cells that are needed as a filler from being clicked.

Below the big character you get as a preview on the right I added the label of the character because there are a large number of "invisible" characters like half-space, non-breaking.space etc.

Hoping you can merge this back.

Regards,
Jelle