Electron: Context menu shows Paste in preview mode

### Expected
Menu only offers Paste when an editable text area was clicked on

Did some research, it's because of this: https://github.com/Automattic/simplenote-electron/blob/fc8921291b6b702db35b076d65e4632203ff4574/desktop/context-menu/index.js#L36

Originally reported over here https://github.com/Automattic/simplenote-electron/issues/1893#issuecomment-703502540

### Observed
Whenever the default Electron menu is shown, Paste is available. This happens if you right-click on the note list, the editor in preview mode, or other parts of the app.

Start in desktop/context-menu/index.js at the referenced line that causes the default Electron menu to appear, then reproduce the menu on the note list, preview-mode editor, and other non-editable areas. Done means Paste is offered only after clicking an editable text area, while the reported contexts no longer show it.

Written by the indexing model from the issue text.