issue #3111: drag transforms/actions from context dialog onto canvas

[mattcasters]

Summary

• Allow placeable create items (transforms/actions) to be click-dragged from the context dialog onto the pipeline or workflow canvas (fixes [Feature Request]: Allow dragging transforms from the "transform selector" directly onto the canvas #3111).
• Hop GUI (native): ghost icon follows the pointer after the dialog closes; place on mouse-up (hop-split supported); Esc / drop outside cancels.
• Hop Web: HTML5/SWT drag-and-drop from the dialog onto the canvas, with display→canvas coordinate conversion for correct placement.
• Plain click placement is unchanged; ALT-Click favorites still work. Tooltip and user docs updated with the shortcuts.

Test plan

• Native Hop GUI — pipeline: open context dialog → click-drag a transform onto the canvas; icon follows pointer; release places it; hop-split when dropped on a hop.
• Native Hop GUI — workflow: same for actions.
• Click (no drag) still places at the original canvas click location; Ctrl-click still opens the editor.
• ALT-Click toggles favorites without closing the dialog; tooltip shows favorites + click-drag hint.
• Hop Web: drag from context dialog onto pipeline/workflow canvas; drop position matches cursor; cancel by releasing outside the canvas.
• Esc cancels an in-progress native placement drag and removes the ghost icon.