gimp_suggest_modifiers(): change "shift_format" and "control_format"
parameters to "extend_selection_format" and "toggle_behavior_format",
which fixes the longstanding problem that the function did the right
thing only by accident.

tools: use gimp_get_extend_selection_mask() instead of GDK_SHIFT_MASK
which is not 100% semantically correct in all cases, but at least a
step in the right direction to make the tool modifiers easier to
improve.