Rework of the Selection of level elements

[deepnight]

This big task serves as a reference for the following features & issues:

• Fix gridless entity moving #704
• Copy selection between levels #712

The goal is to rewrite completely the Selection tool to:

1. support ghost selections (ie. group of elements that behave like level elements, but are detached from the actual level)
2. support copy/cut/paste
3. proper grid-based or pixel-based movement of multiple elements.

Expected behavior:

• create separate "ghost" level to put selected elements into. The ghost is only created if a movement happens, or if a Cut is used.
• "Paste" creates a ghost of the previously copied data: it's not part of the actual level as long the selection exists.