if you have a gdb that can wrap commands (not yet upstream)
then up/down/frame commands will update the source window
when a selected-frame event is available, we can change
the code to use that instead
Clicking in the left gutter of the source window will set a
breakpoint. Currently the effect of this is somewhat invisible,
because there is no way for the GUI to notice when breakpoints
are created.
This adds the "info windows" and "delete window" commands.
"info windows" unfortunately overlaps with the TUI's "info win".
So maybe we should rename this one.
This also adds a new "toplevel" component that knows how to track
windows.
