DocumentPrivate: Add option "Auto Reload Document" to View menu

...and a "Enable Auto Reload" button to the ModOnHdPrompt.

There are a couple of request for such feature with different details and some
opinions against it at all. Not only because KTextEditor is not well suited to
work like 'tail -f'. However, this option should not cause any harm.

Currently jump the view after an update sometimes/often but that may fixed later
e.g. by D17857

- The max update interval is set to 3sec. Shorter intervals would cause a bad
  user experience
- As long as the user scrolls around updates are blocked
- The cursor is set to first/last line of the view when scroll above/below. This
  is currently not the best solution and cause also the mentioned jumps. Setting
  to the middle of the view would reduce this odd jumping but it shouldn't jump
  in any case
- Placing the cursor into the last line cause to work like 'tail -f'

BUG:375361
BUG:377505
BUG:384384
FIXED-IN: 5.57

Differential Revision: https://phabricator.kde.org/D19517