Added a clear mark button. Closes #2.

Text areas expand on focus. Closes #3.

Special criterion added. Marking properties added. Remember last selection.

A bunch of improvements.

Make vertical scroll bars appear. ...

They appear, but I hard to hardcode the text area height meaning
that we now have a fixed size text area, and the vertical scroll
bars are always visible.

The is mostly because of a bug that causes the text areas to grow
horizontally infinitely if you leave it to the layout manager to sort
out.

Initial commit.