Support variable lookup in different frames (especially: "parent" programs)

This one was falsely posted to #20 (closed) but is a different issue, so creating a new one:

The variables has to be resolved looking at the current frame. The following does not work as expected:

• set breakpoint at sub-program (actually main->sub1->sub2)
• continue and reach sub2
• print a variable in sub2
• inspect the backtrace via bt and go up to main, either via up or via frame 1 (code from main is shown)
• use print again to show the same named variable in main

Expected result: variable from main shown Actual result: variable from sub2 shown (as seen in the absolute name)

Possible solution: on "print" check if the current frame is a different than the last one, if yes read the > VARIABLE_STRING_MAIN_COB and then do the printing.