Flint: Support workspace
This MR provides Flint workspace stored inside Redis session.
It provides now:
- A new layout menu to allow to setup predefined layouts
- This menu also allow to lock/unlock the layout
- By default the layout is locked
- Workspace menu to create/remove/switch to workspace
- This allow to store plot, widget, layout referenced by a name
- The layout is not stored (as a "base" workspace) into the Redis session