0.3.1 - Added keyboard shortcuts; Abstracted some user input

Some user input has been abstracted to use Godot's InputMap singleton.

This helps to prepare for multiple input methods, such as computer mice and touchscreen input.

This update also adds basic keyboard shortcuts for undo, redo, opening and saving files, as well as importing PDFs and exporting to CSV.

These basic keyboard shortcuts, coupled with the automatic scene number detection and shot number incrementing from 0.2.0, make ShotLiner a much more useful and usable tool.