This transitions from synchronous IPC calls to asynchronous IPC calls
provided through a synchronous interface in LibFileSystemAccessClient
which allows the parent Application to stay responsive.
It achieves this with Promise which is pumping the Application event
loop while waiting for the Dialog to respond with the user's action.
LibFileSystemAccessClient provides a lazy singleton which also ensures
that FileSystemAccessServer is running in the event of a crash.
This also transitions TextEditor into using LibFileSystemAccessClient.
If a user picks a file which can't be opened for some reason, we should
still return the value, so client applications can report the error
along with the chosen filepath.
Adds new service FileSystemAccessServer which allows programs to
request a file descriptor for any file on the file system.
The user can be prompted to choose the path with a FilePicker, or the
path can be provided by the application which will show a MessageBox
showing the pid and name of the calling process and allows the user to
approve or deny the request.
