Flint: Rework initial curve plot selection (plotselect)

Closes #2065 (closed)

This MR fix and improve the initial selection of the curve plot in order to make it better with previous user selection and the state of the plotselect.

This should improve a lot dealing with curve plot selection.

• Improve the behaviour when plot select and user selection are interleaved
  • plotselect and user selection are timestamped so what we reuse the latest one
• Fix a bug which was resulting on a wrong restoration of the user selection
• Add a tool to display the default selection (go back to plotselect)