Various improvement

From CHANGELOG:

Version 1.3:
- Various bug fixes
- When data with invalid format is passed to the app, it should now inform the user about the error using a dialog box.
- Changed gui-script to purely terminal.
- Rewrote canvases to use OOP design.
- Dialog with preview is used when linear contrast and gamma correction filters are selected.
- Added a license.
- New about page.
- New commands page.
- User no longer has to provide a mode and bitdepth.