Editor enhancements

[Maik13579]

This PR adds following changes to the yasmin_editor:

• Added a real menu bar with File, Add, Edit, View, and Help menus.
• Added Save As.
• Added Open Recent, including a persistent recent-file list and a Clear Recent Files action.
• Added unsaved-change protection: the window title shows when the document is modified, and the editor now asks whether to save before creating a new state machine, opening another one, or closing the window.
• Added undo/redo for editor changes.
• The canvas interaction changed from mainly hand-drag navigation to rubber-band multi-selection; panning is now done with the middle mouse button.
• Multi-selection now works across states, final outcomes, transitions, and text blocks, so whole subgraphs can be handled together.
• Added a right-side shelf panel that can be shown or hidden and used as temporary storage for selected items.
• Added Copy Selection (Ctrl+C): a selected group can be duplicated and then placed manually on the canvas.
• Added drag-and-drop storage/restoration between the main canvas and the shelf, so selections can be moved out of the main graph and restored later.
• Added Extract (Ctrl+E): selected states can be turned into a new nested state machine container.
• Final outcomes can now be reused as multiple visible endpoints for the same logical outcome name instead of being limited to a single visible node.
• These repeated final-outcome views are preserved in XML, including which visible outcome a transition points to.
• Editing a final outcome now supports renaming it as well as changing its description.
• Placement of newly added items is smarter and tries to use a nearby free position instead of stacking items on top of each other.
• Multiline text content is saved and loaded more reliably.
• Name-conflict handling is stricter: child-state names and final-outcome names are checked against each other more consistently when adding or renaming items.

I started adding some tests, but it's far from being fully covered.

[Maik13579]

I also added GUI tests. The Jazzy CI error is unrelated to yasmin_editor, the failing tests are in yasmin_ros (test_action_client_state / test_action_client_state_cpp), so this does not seem to be caused by the editor changes.