New Feature: Specify a filename in the file dialog path api

[mlabbe]

Specifying a filename in the save autocomplete path invalidates the path.

  The following behaviours occur:
   - Windows: Path fails regardless of whether the file exists, goes to Documents instead.
   - Linux:   Goes to the directory if the file does not exist, completely fails if the file exists with an
     anonymous hard drive icon.
   - MacOS: Path fails unless the file exists, in which case it becomse the default path.  Saving to this path
     makes Untitled.png show up.

  Expected results:
   - file does not exist: go to the path
   - file does exist: go to the path and autocomplete the filename if the OS supports it

There are no plans to polyfill missing OS functionality.