Issue #998: Allow copy/paste and context menu in all card windows.

[Ticket #998](https://anki.lighthouseapp.com/projects/100923/tickets/998-copying-text-from-preview-window)

The context menu was purposely being ignored if we aren't in the reviewer. Instead of the mw state, I used a flag in AnkiWebView to decide what we should do. The flag can be specified when the object is constructed or toggled manually. The toggle was required because we aren't creating a new AnkiWebView for the reviewer, but we do know when we go into and out of it.

The copy with Ctrl+C simply needed the focus mode to not be ```Qt.NoFocus```. I made it so the focus mode is also set if the above flag is made True.

This change makes the template editor's preview windows also support copy/paste and a context menu.