Feature: #111149 - Remember the chosen workspace preview mode per user 

See forge#111149

Description 

The workspace preview in the backend offers three modes to compare the workspace version of a page with its live version: a slider between both versions ("slider"), and the two versions stacked vertically ("vbox") or side by side ("hbox").

The mode a backend user selects is now stored in the backend user settings under workspaces.previewMode and used as the initial mode when the workspace preview is opened again. Previously, the preview always started with the first mode of the page TSconfig option workspaces.splitPreviewModes .

If the stored mode is not allowed for the page by workspaces.splitPreviewModes , the first allowed mode is used as before.

Impact 

Backend users who prefer to compare the versions side by side or stacked no longer need to switch the preview mode each time they open the workspace preview.