Fix dock and message tray behavior

[kyuucr]

Fixes #41. Dock and Message Tray should be happily coexist now. Dock is always hidden when Message Tray is showing, and when Message Tray is hiding, it will readjust (showing/hiding) based on intellihide. Message Tray won't be opened when mouse is hovering the Dock.

One problem though, when Dock is open and hovered on, pressing Super+M will open Message Tray and the Dock will be stuck open until hovered out. Can't fix it yet.

[me4oslav]

Should I merge without the fix or no?

[kyuucr]

I don't know, how do you feel about it? For me, I don't really want to fix this too much since the message tray will be nuked in 3.16 after all.

[tomvanderlee]

I would say, don't fix at all and wait for gnome 3.16.

[kyuucr]

I concur, this fix removes the hassle of accidentally opened message tray when trying to open dock and it's the better experience for me. I just don't want to fix the Super+M shortcut behavior.

For the concern of when 3.16 released and how do we deal the message tray at that point, I already have a plan. We can implement some kind of checking before adding message tray signals, something like:

this.signals.push(
    // the usual signal register
    [
        ...
    ]
);
// checking for message tray
if (messageTray is exist) {
    this.signals.push( /* messageTray signals */ );
}