Webclient: refactor menus, pt.4 (#532, #534).

The canvas context menus do scroll when invoked near screen bottom.

DEVELOPER NOTES

DM's GUITookit now supports context menus.
{{{
dm4c.ui.context_menu()
}}}
All DM menus are now unified and based on GUITookit.
The proprietory canvas context menu code is dropped along with all menu related CSS.
The GUIToolkit menus and context menus rely on the jQuery UI Menu widget.

Menu refactoring is basically done.
Minor issues remain.

BREAKING CHANGE

All menu selection handlers are unified and receive 3 arguments:
    menu_item - the selected menu item
    x, y      - coordinates of the selecting mouse click

This applies to toolbar menus as well as context menus.
If your plugin provides topic/association/canvas commands resp. places commands in the toolbar menus it must be possibly adapted. Check your command handler arguments.

See #532.
See #534.