Webclient: combine 1- and 2-click gesture (#534).

Both, the "traditional" 2-click gesture and the new 1-click gesture can be used together.

Each time you use a menu you can do both:
- work just like before, that is click to open a menu, and do another click to select an item.
- or save 1 click by holding the mouse button down, move to the item, and release the button.

This works for both, context menus and stationary menus.

The 2-click gesture is still useful in case of long menus, which are scrollable.

I think, we have the best of "2 worlds" now.

DEVELOPER NOTES

GUIToolkit:
`dm4c.ui.button(config)` accepts another config option:
    `on_mouseup` -- the handler fired on button mouseup (a function).

See #534.