Lasso/box selection switching and usage over existing layers

We want to update dragging and selection such that:
- Ctrl+drag's lasso selection, upon releasing Ctrl, switches to a regular box selection, and back to lasso by holding Ctrl again. It draws a straight line between the cursor position last time the lasso was removed and when it's now restored, allowing the user to also use this as a technique for drawing straight line sections of the lasso, but also just to decide to use box selection instead of lasso at any time.
- Ctrl+dragging an *object* currently drags its layer(s) but we want to make it instead always lasso select. This lets the user perform a lasso selection over an existing layer, or a box selection but releasing Ctrl as in the previous bullet point.
- Lasso selection has a bug that should be fixed: aborting the lasso selection (RMB or Esc) then creating a new lasso selection ends up erroneously extending the previously drawn lasso path because abort fails to clear the lasso data (which also wastes memory!). (#2792)

El issue no menciona archivos, pruebas ni puntos de entrada, así que empieza rastreando el manejo de drag, lasso, box-selection, Ctrl, RMB y Esc. Se considera completado cuando Ctrl cambia de forma fiable entre lasso y box selection, Ctrl-dragging sobre objetos existentes realiza una selección en lugar de arrastrar la capa, y los datos de lasso abortados se limpian antes de la siguiente selección.

Escrito por el modelo de indexación a partir del texto del issue.