Webclient: refactor menus, pt.1 (#532).

The DM GUI Toolkit's menus rely on the jQuery UI Menu widget now.

See #532.
Webclient: refactor menus, pt.2 (#532, #534).

Long menus do scroll.
The scroll wheel can be used.

The main toolbar's menus reveal all their items without scrolling.

See #532.
See #534.
Webclient: refactor menus, pt.3 (#532).

GUITookit: the Menu's generic parts are factored out into new class BaseMenu.
This will be the base class for the upcoming context menus as well.

See #532.
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.
Webclient: fix long menus (#532, #534).

In long menus, when a (vertical) scrollbar appears:
    - the menu items are not line wrapped (the menu is wide enough).
    - no horizontal scrollbar appears.

=> If one or both is not true for your OS or windowing system please send me an unscaled screenshot of the menu.

Note: you can use the mouse wheel or whatever scrolling gesture to scroll through long menus.
The former upper/lower "scrolling areas" are gone.

See #532.
See #534.