Port file delete trash operation to Vala

[jeremypw]

Fixes #137

Continuing the work of porting remaining C code to Vala.

Also introduces a rudimentary FileOperation Manager to keep track of what jobs are running and in future make this information available to the UI for purposes other than progress indication (e.g. locking the appropriate unmount buttons or providing information to warning dialog).

• Port permanent delete file
• Port try trash file
• Port progress interaction
• Port undo interaction
• Port confirm delete (or not) logic
• Port delete/trash folder
• Ensure errors and exceptions handled correctly
• Improve commenting

marlin_file_operations_copy_move_link no longer handles operations with trash:/// as a destination. For the present, testing whether the destination is trash is moved into the calling code when necessary. However, once copy/move/link is ported to Vala (in a future PR) that will not longer be required.

Using async functions for trash and delete fixes some problems with the progress window not appearing.