(gdba-marker-filter, gdb-output-burst): Merge and

(gdba-marker-filter, gdb-output-burst): Merge and
rename gud-gdba-marker-filter.
(gdb-append-to-inferior-io): Only pop up IO buffer if there is
output.
(gdb-make-instance): Remove (put into gdba).
Use gdb-instance-enqueue-input instead of
gdb-instance-enqueue-idle-input for user functions.
(gdb-instance-target-string): Simplify.
(in-gdb-instance-context): Remove.
Expand Commentary.