API for launching or switching a particular app ID

[danirabbit]

Problem

For the new Dock, we can't handle any window management functions to be compatible with Wayland. However, in our recent UI study we found that users expect to be able to switch to a running app and switch between windows of running apps in the dock.

Proposal

To facilitate these features, we need a need API in Gala to handle launching and switching since the dock isn't aware of open windows etc. The simplest way forward here would be a dumb API from the Dock's perspective where we only send an app ID that we would like Gala to act on.

If there are no open windows associated with that app ID, Gala should launch the appinfo associated with that ID.

If there is one open window associated with that app ID, Gala should switch to that window, across workspaces if necessary.

If there are multiple open windows associated with that app ID, Gala should show them in a window spread (like the multitasking view)

Prior Art (Optional)

I'm fairly certain this is how the Ubuntu Unity launcher worked, but I don't have any evidence of that on hand