gladevcp -fix hal_gremlin's highlight line behaivor

clicking a line in gremlin now sends a GObject signal of
the line number, so a program can catch and do something.

Hal_gremlin does this be overriding the new glcannon function
that updates the highlighted line number. It adds the emitting
of a signal.
gscreen -add gcode highlight-by-graphics-click to Gaxis

This is an example of Gremlin's new highlighting ability.
click a line in the grahics plot and it will highlight the line
in the gcode. This mirrors AXIS's behaivor.