Travis CI used old cache and did not find the compilation
errors (instead it that errors has been found by us).
Also get rid of some Qt private parts, more fwd decls and so on.
Some major platform-dependent file operations refactoring.
All methods like "open file", "open file with", "show in folder"
were moved to core/file_utilities module with platform-dependent
backends. All methods interacting with DesktopServices made async.