Add directory picker to web target

[timo-drick]

Adding support for a directory picker.
It is still based on the input field with the webkitdirectory attribute:
https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/input#webkitdirectory
https://developer.mozilla.org/en-US/docs/Web/API/File/webkitRelativePath
It is supported on most modern browsers.
Therefore a change in the PlatformFile was necessary. I created an interface.
The current implementation creates virtual directories because the input form returns a list of all files recursively from the user selected directory. Than a virtual tree is build in memory and linked to a virtual root directory.
Moved following PlatformFile extension functions to common:

• path
• parent()
• isRegularFile()
• isDirectory()
• list()

I am also experimenting with FileSystemAccessApi. Which does allow to support more real access to the host file system. Including write access and also maybe bookmarking of resources (Limited to chromium based browsers).
Other than that it is maybe also possible to support a sandbox filesystem based on thr File System API:
https://developer.mozilla.org/en-US/docs/Web/API/File_System_API
Than web Filekit could also provide this files and cache directory with read/write access.
I think what maybe missing in the PlatformFile is an extension function where you can check if the File/Directory is writeable.
(And also maybe if a file/directory do has read access allowed?)