qbs should be fully case sensitive

qbs is currently in case-insensitive in many places.
this is no particularly good idea for many reasons:

• it doesn't really fit into everything else in the project (c++, qml, ...) being case-sensitive
• it becomes hard to stay consistent in the docs, examples, actual implementation, etc., because the case becomes pure convention which is not checked anywhere
• the implementation is slower (e.g., on case-sensitive file systems, need to list directories to find files) and complex (e.g., hashmaps need special (slow ...) hash functions)

it is arguable that case insensitivity on the command line is convenient, but the inconsistency this introduces is not really worth it.