Add a new button in view toolbar to save view content to clipboard or file

A new 'Screenshot' button is added to the right of the Undo/Redo buttons. It can also be accessed through the new item Copy Screenshot to Clipboard item in the Edit menu.

When clicked without pressing a keyboard modifier (Ctrl, Alt or Shift), the capture is performed to the clipboard. Otherwise, the button/menu acts as a shortcut to File/Save Screenshot (to a file).

Note that this feature is only available for Render and Charts views.