Make Workspace process batched output updates

Currently the Workspace processes output updates as they occur, e.g.
when the drm backend scans connectors, the Workspace will handle
hotplugged outputs one by one or if an output configuration changes the
mode of several outputs, the workspace will process output layout
updates one by one instead of handling it in one pass. The main reason
for the current behavior is simplicity.

However, that can create issues because it's possible that the output
layout will be temporarily in degenerate state and features such as
sticking windows to their outputs will be broken.

In order to fix that, this change makes the Workspace process batched
output updates. There are several challenges - disconnected outputs have
to be alive when the outputsQueried signal is emitted, the workspace
needs to determine what outputs have been added or removed on its own.