Add block elements for freeform mode

[guillaumecabanel]

It would be nice to draw in freeform mode with block elements like ▉ (U+2588), ▆ (U+2586 ) or ▒ (U+2592).

[porg]

I propose a generic an hence very powerful solution for this:

At the bottom of the character selection menu, insert a text field with the label "Use this custom character" which can take a single character, any single-width unicode character that is supported.

I made a mockup: