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

bitcoin-qt.pro

@@ -100,7 +100,7 @@ HEADERS += src/qt/bitcoingui.h \
     src/qt/optionsdialog.h \
     src/qt/sendcoinsdialog.h \
     src/qt/addressbookpage.h \
-    src/qt/messagepage.h \
+    src/qt/signverifymessagedialog.h \
     src/qt/aboutdialog.h \
     src/qt/editaddressdialog.h \
     src/qt/bitcoinaddressvalidator.h \
@@ -160,8 +160,7 @@ HEADERS += src/qt/bitcoingui.h \
     src/qt/qtipcserver.h \
     src/allocators.h \
     src/ui_interface.h \
-    src/qt/rpcconsole.h \
-    src/qt/verifymessagedialog.h
+    src/qt/rpcconsole.h
 
 SOURCES += src/qt/bitcoin.cpp src/qt/bitcoingui.cpp \
     src/qt/transactiontablemodel.cpp \
@@ -169,7 +168,7 @@ SOURCES += src/qt/bitcoin.cpp src/qt/bitcoingui.cpp \
     src/qt/optionsdialog.cpp \
     src/qt/sendcoinsdialog.cpp \
     src/qt/addressbookpage.cpp \
-    src/qt/messagepage.cpp \
+    src/qt/signverifymessagedialog.cpp \
     src/qt/aboutdialog.cpp \
     src/qt/editaddressdialog.cpp \
     src/qt/bitcoinaddressvalidator.cpp \
@@ -220,7 +219,6 @@ SOURCES += src/qt/bitcoin.cpp src/qt/bitcoingui.cpp \
     src/qt/notificator.cpp \
     src/qt/qtipcserver.cpp \
     src/qt/rpcconsole.cpp \
-    src/qt/verifymessagedialog.cpp \
     src/noui.cpp
 
 RESOURCES += \
@@ -229,7 +227,7 @@ RESOURCES += \
 FORMS += \
     src/qt/forms/sendcoinsdialog.ui \
     src/qt/forms/addressbookpage.ui \
-    src/qt/forms/messagepage.ui \
+    src/qt/forms/signverifymessagedialog.ui \
     src/qt/forms/aboutdialog.ui \
     src/qt/forms/editaddressdialog.ui \
     src/qt/forms/transactiondescdialog.ui \
@@ -237,7 +235,6 @@ FORMS += \
     src/qt/forms/sendcoinsentry.ui \
     src/qt/forms/askpassphrasedialog.ui \
     src/qt/forms/rpcconsole.ui \
-    src/qt/forms/verifymessagedialog.ui \
     src/qt/forms/optionsdialog.ui
 
 contains(USE_QRCODE, 1) {