WIP: Allow to limit max lines output in multiline

Infinite multiline ends in a tiny almost invisible letter.

screenkey --bg-color '#fff' --font-color '#000' -g 400x400+0+0 -s large -p fixed --multiline --max-lines 4

Other key monitors limit the number of lines exposed.

It is a wip because this has some problem:

• unknown characters if you type too fast

• from n lines exposed, n-1 lines could be still showed, instead of clean all
• reduce the font size as lines appear seems confusing, maybe all lines should be printed all the time