updated readme file (converted markdown image-list to HTML table, to make sure the layout is stable)

cleaned up the repository (application code, shader code, resource folder, improved gamepad controls, enabled geometry instancing, added various (small) gameplay changes, fixed various issues)

changed level design image type from XCF to PNG (it does not use XCF features, and can be previewed online as PNG)

changed some plate values at the beginning (to better introduce the plate mechanic and reduce confusion on the first puzzle)