Fixed handling of inventory in creative mode (normal inventory is not trashed anymore), fixed mese pick speed, added some forgotten stuff

- When flowing liquid starts to flow down, it stops contributing to its neighboring nodes' liquid level (possibly transforming it back to air)
- Flowing liquid turns into a source if there are at least 2 adjacent liquid source nodes
- If a new liquid flow reaches existing flowing liquid, the existing liquid is now updated properly.

- When flowing liquid starts to flow down, it stops contributing to its neighboring nodes' liquid level (possibly transforming it back to air)
- Flowing liquid turns into a source if there are at least 2 adjacent liquid source nodes
- If a new liquid flow reaches existing flowing liquid, the existing liquid is now updated properly.

New map generator added (and SQLite, messed up the commits at that time...) (import from temporary git repo)

Moved some mapnode content stuff from mapnode.{h,cpp} and digging property stuff from material.cpp to content_mapnode.{h,cpp}