Create new window on activate

### Problem

We want to be able to consistently create new windows on middle click for multi-window apps as in https://github.com/elementary/dock/issues/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 https://github.com/elementary/dock/issues/77#issuecomment-1821781386

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_