Rework EngineInstance::generatePreview, add preview cleanup function in enginelib
Add possibility to instantiate HWEngine objects from QML, reorganize work with preview
Rename protocol_version to hedgewars_engine_protocol_version to ensure use of correct library
Move the static mines test (
bug #96) to a separate folder to allow using all tests in tests/lua for release testing.
ACF1: Fix Lua error when hitting Attack after failing the rope challenge
"difficulty" variable was falsely set to 0 in this case, but only 1 or 2 are allowed