[file_selector] Add Linux-specific Dart implementation

[stuartmorgan]

Part 2 of prep for moving this to flutter/plugins:

• Adds a Linux Dart implementation based on the shared method channel implementation.
  • Renamed the channel, as usual for internal method channels.
  • Added the assertion for unsupported type groups, matching the other platforms.
  • Moved some logic for converting type groups to the format needed by the GTK APIs from C to Dart.
  • Added more use of string constants for ease of keeping Dart and C in sync.
• Updates the example with a recent copy from another desktop implementation.

[stuartmorgan]

Individual commits are probably easier to review here.