Add new enum GimpToolActiveModifiers { OFF, SAME, SEPARATE } and
new API gimp_tool_control_set,get_active_modifiers(), the default
value is OFF.

OFF: the tool gets no modifier keys at all during a stroke

SAME: all modifiers are always delivered via GimpTool::modifier_key(),
and no magic is applied whatsoever when a mouse button is pressed or
released.

SEPARATE: this is the "classic" way: modifiers while hovering and
while stroking are delivered separately, and hover modifiers don't
affect stroke modifiers.