GUI: merge sign/verify message into a single window with tabbed UI

- add UI-feedback via QValidatedLineEdit
- copy button for generated signature was moved to the signature output field
- add an addressbook button to verify message tab
- input fields are now evenly ordered for sign and verify tabs
- update FIRST_CLASS_MESSAGING support to ensure a good UX
- add a button and context menu entry in addressbook for verify message (to be consistent with sign message)
- focus is now only set/changed, when clearing input fields or adding an address via addressbook
- re-work / update some strings
- ensure model gets initialized in the SignVerifyMessageDialog constructor
- add checks for a valid model to both addressbook buttons
- remove unneeded includes for Qt GUI elements that are listed in ui_signverifymessagedialog.h anyway

src/qt/addressbookpage.h

@@ -60,6 +60,7 @@ private slots:
     /** Copy address of currently selected address entry to clipboard */
     void on_copyToClipboard_clicked();
     void on_signMessage_clicked();
+    void on_verifyMessage_clicked();
     void selectionChanged();
     void on_showQRCode_clicked();
     /** Spawn contextual menu (right mouse menu) for address book entry */