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.
Left: auto update, privacy and security, local storage box +
some minor things, like design fixes, codes like loadlang and
clearing of the local storage (box) and temp download folder.
