Improve and simplify commands/menus/keybindings contributions

[akosyakov]

• [ ] remove 1-n between command and handlers, by simply let put all the functions of CommandHAndler into Command
• [ ] remove keybinding context stuff and totally rely on command’s isEnabled()
• [x] allow passing args to commands, as varargs
• [ ] use T | Promise<T> for execute, isEnabled and so on - https://github.com/theia-ide/theia/issues/435
• [ ] move isVisible to a menu item
• [x] allow dynamic menu contributions
• [ ] expose API to build menus from MenuModelRegistry, like findGroup
• [x] consider introducing a composite contribution which allows contributing commands, menus and keybindings at once - one can implement all contributions with one class
• [x] move keybindings to browser since it relies on dom events - https://github.com/theia-ide/theia/issues/434
• [x] rename modifiers: https://github.com/theia-ide/theia/issues/458

Add more points if I miss something.

[svenefftinge]

We should keep the KeybindingContext (and registry). It nicely allows to contribute contextual keybindings, especially for users through keymaps.