Small fixes

[abakkk]

The last commit is completely optional. It can be removed.

retrieve the default cursor when the menu is open:

• Fixes #7.

menu background:

• Keep the popup-menu-boxpointer style class so the background is not transparent.

menu key:

• Open the menu on 'Menu' key pressed. Fixes #8.

do not end picking twice:

• Since a388bde, the color area handles picking ending when a color is selected from the menu.

destroy:

• Call Clutter.Actor.prototype.destroy (super.destroy) to really destroy the area.
• Use PopupMenu.prototype.destroy to destroy the menu actor. It must be called before destroying area._icon, which is its source actor.

clean:

• removeChrome is useless because the layout manager takes care of untracking destroyed actors.
• Do not bother with signal handlers and use virtual functions.

[tuberry]

Thanks! Patch merged.