1. fix possible empty layers (caused by continues walls)
2. fix normal node may be absorbed in drop_nodes in "tree hybrid"
3. popup message to ask the user to select best parameters for tree strong
and tree hybrid.
4. auto select lightning infill for tree hybrid.
Change-Id: I7d172887a9b0e4f268267684e83fc02dc7ec6a0c
(cherry picked from commit bc6d130e86c252d7fc1f84c4df7da95050dd5d51)