Reimplement linux file picker using native APIs in the app rather than calling out to zenity

[Hixie]

The current approach requires that an additional package be installed for the application to work, and causes a new system process to be spawned for the dialog that is separate from the actual app. This is particularly problematic in scenarios where the system is locked down to prevent applications from spawning processes.

There doesn't seem to be any reason why we could implement this using regular message channels to code implemented natively in the app, like a normal plugin.