docking: fix window picker space for multi-monitor environments

[taoky]

Attempted to fix https://github.com/micheleg/dash-to-dock/issues/2154.

This PR:

• Checks whether the dock is in primary monitor in _computeWorkspacesBoxForState. It looks like that this function only cares about the workspace on primary monitor, as original GNOME dock is in primary one.
• Adds a hook for WorkspacesView.SecondaryMonitorDisplay._getWorkspacesBoxForState. This function is similar to _computeWorkspacesBoxForState, and looks like the correct function to hook without glitches.

In _getWorkspacesBoxForState:

• maybeAdjustBoxSize is not reused as the workspace in secondary monitor does not have things like searchBox.
• Only bottom dock is handled, as it looks like nothing is wrong with docks positioned at left/right, and... I'm not sure how dock at top shall look like.

• preferredHeight might be too large here:

  

  But there's no this.spacing inside St.Widget :(

And this only fixes the window picker part. A similar bug in multi-monitor environments, when "Workspaces on primary display only" is chosen, still exists when overview state is app grid -- but this could be triggered even without any extensions and thus a gnome-shell bug at present (https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5316), though the space between dock and workspace is still too small.

[3v1n0]

Oh, thanks for looking into this!

Let's see if #2166 causes this to break before merging