Add copy to PNG option on context menu (#941)

* Add copy to PNG option on context menu

* lint & refactor & fixes

* add keybinding

* swap keybinding

* fix docs

Co-authored-by: dwelle <luzar.david@gmail.com>

src/locales/en.json

@@ -3,6 +3,7 @@
     "paste": "Paste",
     "selectAll": "Select All",
     "copy": "Copy",
+    "copyAsPng": "Copy to clipboard as PNG",
     "bringForward": "Bring Forward",
     "sendToBack": "Send To Back",
     "bringToFront": "Bring To Front",