Introduce force-keep-workspaces-alive config option

[nikitabobko]

force-keep-workspaces-alive = ['A', 'B', 'C', 'D']

The mental model is described here: #875 (comment)

The workspaces in the menubar should be sorted the same as workspaces in the force-keep-workspaces-alive array (to cover #830)

To make all workspaces "phantom" (#996), one can use the empty array:

force-keep-workspaces-alive = []