Support curve multi-selection in Transform Editor

[MichelJansson]

We often need to apply or remove the same transforms on many curves which has to be done one-by-one. This is tedious and error prone and multi-select makes intuitive sense, so this is my proposed solution.
Thanks!

Enhanced plotwidget transforms (Transform Editor) to support curve multi-selection, enabling batch operations of curve transforms.

• Updated curve list to handle multi-selection.
  • Changed curve selection mode to ExtendedSelection.
  • When multi-selection is started, the transform selection UI is cleared to make batch manipulation explicit (ie. batch transform not applied until explicitly selected).
• Changed transform list for multi-curve updates.
  • Transform state is applied to all selected curves using existing XML serialization api.
  • Alias logic updated to better stay in sync with currently selected transform unless manually changed by checking it the current alias is the default alias for the previous transform.