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