Krusader codebase restructuring

When the repository is cloned, it is located in "krusader" dir. This leads to strange-looking paths like krusader/krusader/main.cpp, because there is "krusader" subdir with file manager source code, and confuses inexperienced users when they try to build a version from the repo. Renaming the subdir to "app" mitigates the problem.

The change preserves git history (i.e. git blame works correctly on all files within "/app").

This kind of change should be done before the release, otherwise it will complicate bugfix backporting a lot.

See the proposal on further krusader codebase restructuring in the discussions.