Using the following command gives unexpected results, and has no easy
replacement:

%system "xterm  -geometry %50x50+25+25"

This is because xterm interprets -geometry height and width in units
of characters.  So mgp should not rely on this to enforce a size -
other apps may use the same convention.

A good replacement would be to grab the window as a window-manager
would, and put it in a sub-window of the mgp window, on which we'll
have perfect control.  As this is a different functionnality from what
%system does, maybe a new command has to be created.