workspacesView: Use side-by-side layout

workspacesView: Use side-by-side layout

Currently, WorkspacesView positions each workspace on a
per-page layout, each page with the allocated width and
height of WorkspaceView. This layout doesn't work well
with horizontal workspaces.

Layout workspaces side by side, instead of per page. The
layout is influenced by a "fit mode", which reflects the
different behaviors exposed in the mockup. This fit mode
represents whether a single or all workspaces will fit
available geometry.

The single fit mode is always used for now. Next commits
will make it switch to the all fit mode when in the app
grid state.

The translation_{x,y} also needed to reflect the switch to
a side-by-side layout, and use the geometry of the workspaces
to determine the offset. Notice that, when the fit mode is ALL,
there's no translation applied.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1613>