"Quality of life" improvements + bugfixes

Added "export to JSON"/ "import from JSON" .

(How to save your construction as a sketch file => In settings, select "Toolset" = full, then on the toolbar you will see a V-shaped button. Click on the button, it opens a popup menu.  If you select "Export to JSON", then your browser downloads "sketch.json".)

Added the ability to take screenshots.