Create new window on activate

[danirabbit]

Problem

We want to be able to consistently create new windows on middle click for multi-window apps as in elementary/dock#77

But currently activating files presents the existing main window

Proposal

The new dock now handles focusing if an app has a single window, so we should make activate open a new window. That way we can have middle click always activate as suggested in elementary/dock#77 (comment)

We should also add SingleMainWindow=false in the desktop file while we're here to make sure we're hinting to the shell about our multi-window behavior

Prior Art (Optional)

No response