Draft: Node insertion and removal modes

This MR adds two special modes to the Node Tool, both activated by pressing and holding configurable modifier keys.

Node insertion mode

Default modifier: Ctrl. In this mode, the mouse cursor has a little "plus" on its side. A round handle appears which follows the mouse cursor, but remains constrained to the selected paths. Clicking on this handle inserts a new node at that position. The handle is subject to snapping, if enabled.

Node deletion mode

Default modifier: Alt. In this mode, the mouse cursor has a little "minus" on its side. You can delete nodes, tangent handles and path segments by simply clicking on them.

demo

Additionally, the context menu of the Node Tool allows the user to change the type of the selected nodes.

Related feature requests: inbox#4329, inbox#3702