new utf decoder

This patch replaces current utf decoder with a new one, which is ~50
lines shorter and should be easier to understand. Parsing 5 and 6
sequences, if necessary, requires trivial modification of UTF_SIZ
constant and utfbyte, utfmask, utfmin, utfmax arrays.
Fix segfault when pressing PrintScr without a selection
Fixing a compiler warning with the assignment.
There is no need for a full dirt redraw.
Updating the TODO in preparating for 0.5.
0.5 release.
Merge branch 'master' into unstable-argb