VOXEDIT: implement brushes

[mgerhardy]

Find downloads of the brush feature builds attached to the actions of the pull request. Feedback and testing is much appreciated. Please attach comments here at this PR or join the discord server

A video of the current state (2023-08-27)

Tasks

• [x] Add brush panel (remove ModifierPanel and ScriptPanel)
• [x] Convert script handling into brush (ScriptShape)
• [x] Extract shape related code into ShapeBrush class
• [x] Extract plane modifier mode into PlaneBrush
• [x] Move most of the member variables into the appropriate brush classes
• [x] Create a StampBrush that allows to use existing models (e.g. from AssetPanel via drag&drop) as a stamp to place them at the cursor position.
• [x] StampBrush: add button for replacing colors with a single voxel
• [x] StampBrush: File->New screws up the stamp brush preview and selected model (maybe reset() related)
• [x] StampBrush: allow to create a stamp from the current selection
• [x] Add unittests for brushes (See ModifierTest)

Fixes https://github.com/mgerhardy/vengi/issues/130

[mgerhardy]

8583d220c7a67d5008da2b8e478622c3eeefd058 reverted the decision to put the script integration into a brush