codenav/goto_file: Improvements/fixes (tab key autocomplete, absolute dirs, ~/)

PR geany#1390

New features & changes:

- The Tab key can now be used to:
  (Inspired from shells, and GtkFileChooser)
	- Show the completion list when it's hidden
	- Trigger an inline completion
	- Accept a suggested inline completion
- Keep dialog open after user denies creation of not-found file (geany#1329)
- Allow absolute paths, and recognize ~ as the user's home dir
- If no document is open, the current dir is the user's home dir
- Show error if the entered path is a directory
- Bug fixes, improvements, code formatting fixes

Author: gkatev