Mark strings for translation.

Mark strings for translation.

The procedure [mc ...] will translate the strings through msgcat.
Strings must be enclosed in quotes, not in braces, because otherwise
xgettext cannot extract them properly, although on the Tcl side both
delimiters would work fine.

[jes: I merged the later patches to that end.]

Signed-off-by: Christian Stimming <stimming@tuhh.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>