There can be cases when one wants to open a file not just for reading
but also for writing. It was already possible to have a custom open mode
for request_file, now it is also possible through the open/save dialogs.
This commit adds a new request to the FileSystemAccessServer
endpoint, allowing the clients to get read-only access to a file
without getting a Dialog-box prompting the user for access.
This is only meant to be used in cases where the user has asked
specifically to open a file through the command-line arguments.
In those cases, I believe it makes sense for the read-only access
to be implicit. Always prompting the user gets a bit annoying,
especially if you just quickly want to open a file through the CLI.
The new request name has been made extremely specific to make sure
that it's only used when appropriate.
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.
