* app/tools/tools-enums.[ch]: add enum GimpButtonPressType which can
be { NORMAL, DOUBLE, TRIPLE }

* app/tools/gimptool.[ch]: add press_type paramater to GimpTool::button_press()

* app/tools/gimp*tool.c
* app/tools/tool_manager.[ch]: changed accordingly.

* app/tools/gimptoolcontrol.[ch]: add members and API so tools can choose
to receive double and triple clicks.

* app/display/gimpdisplayshell-callbacks.c (gimp_display_shell_tool_events):
dispatch double and triple clicks to tools if they want them, and if they
became active by the preceding normal button press.