gdb: whitespace changes
gdb: defend against use-after-free of breakpoint structures

This should prevent the crash observed in https://paste.kde.org/p0vezsuve
Since I am unable to reproduce that crash, it is unclear whether the scenario
that lead to the crash is handled completely correctly now.

REVIEW: 121481
gdb: use Q_SLOTS instead of slots
gdb: remove dead references to a GDBBreakpointWidget
test_gdb: use Breakpoint::state() instead of IBreakpointController::breakpointState()
fixed errors introduced by merge