[Qt] New status bar Unit Display Control and related changes.

- New status bar control shows the current Unit of Display.
  When clicked (left,or right button) it shows a context menu
  that allows the user to switch the current Unit of Display (BTC, mBTC, uBTC)
- Recent Requests and Transaction Table headers are now updated when
  unit of display is changed, because their "Amount" column now displays the
  current unit of display.
- Takes care of issue #3970 Units in transaction export csv file.
- Small refactors for code reuse.
- Demo Video https://www.youtube.com/watch?v=wwcr0Yh68go&list=UUG3jF2hgofmLWP0tRPisQAQ
This looks pretty similar to ``QString TransactionTableModel::getAmountTitle()``, perhaps you can add a generic function somewhere (e.g. GUIUtil)?