Issue #7114 : Add optional Spoon-style palette tree to pipeline and workflow graphs

[mattcasters]

Fixes #7114.

Adds an optional left-hand Spoon/PDI-style palette tree on pipeline and workflow graphs so people migrating from Kettle can pick transforms and actions without relying only on the canvas context dialog.

• Toolbar button (hidden by default, remembered in hop-config.json) shows a categorized tree with miniature icons, tooltips, filter, expand/collapse, and clear filter
• Favorites category at the top of the tree
• Drag onto the canvas to place; Shift-drag hops from the selected or last chained item
• Shift-double-click adds the clicked palette item 150px to the right of the selected/last chained transform or action, hops to it, and selects the new item so the next one continues the chain
• Respects the Design for engine filter; Alt-click and the tree context menu toggle favorites
• Space and other printable keys type into the palette filter instead of firing graph shortcuts

The existing context dialog is unchanged.