Added support for a configuration file, `.hkeplot`.

Utilized the configuration file to make a few enhancements.

Closes #7. Configuration file now stores the default locations for searching for files.
Closes #13. `GraphFrame` now has a style that includes the title bar, so it can be easily dragged for movement again.
Closes #14. Configuration file now stores the loaded files and their configurations. Does not yet support memory of channel-specific labels, but does support file-specific register sources.