Webclient: 1-click menus (#534).

The menu gesture has fundamentally changed: now we have 1-click menus.

Before:
{{{
Click ---------> Choose ---------> Click
}}}
That is 2 mousedowns and 2 mouseups = 2 clicks

Now:
{{{
Mousedown ---------> Choose ---------> Mouseup
}}}
That is 1 mousedown and 1 mouseup = 1 click

So the effort of the most frequent usage gesture, a menu selection, has reduced by half.

(That's the very gesture I'm familar with by using a Mac.
BTW: how it works on Gnome, KDE, Unity, ...?)

-> What do you think? Do we want keep it this way?

I'm quite happy until it comes to scrolling long menus.

DEVELOPER NOTES

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

See #534.