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/forms/addressbookpage.ui

@@ -6,7 +6,7 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>627</width>
+    <width>691</width>
     <height>347</height>
    </rect>
   </property>
@@ -17,10 +17,10 @@
    <item>
     <widget class="QLabel" name="labelExplanation">
      <property name="text">
-      <string>These are your Bitcoin addresses for receiving payments.  You may want to give a different one to each sender so you can keep track of who is paying you.</string>
+      <string>These are your Bitcoin addresses for receiving payments. You may want to give a different one to each sender so you can keep track of who is paying you.</string>
      </property>
      <property name="textFormat">
-      <enum>Qt::AutoText</enum>
+      <enum>Qt::PlainText</enum>
      </property>
      <property name="wordWrap">
       <bool>true</bool>
@@ -99,7 +99,7 @@
      <item>
       <widget class="QPushButton" name="signMessage">
        <property name="toolTip">
-        <string>Sign a message to prove you own this address</string>
+        <string>Sign a message to prove you own a Bitcoin address</string>
        </property>
        <property name="text">
         <string>&amp;Sign Message</string>
@@ -110,6 +110,20 @@
        </property>
       </widget>
      </item>
+     <item>
+      <widget class="QPushButton" name="verifyMessage">
+       <property name="toolTip">
+        <string>Verify a message to ensure it was signed with a specified Bitcoin address</string>
+       </property>
+       <property name="text">
+        <string>&amp;Verify Message</string>
+       </property>
+       <property name="icon">
+        <iconset resource="../bitcoin.qrc">
+         <normaloff>:/icons/transaction_0</normaloff>:/icons/transaction_0</iconset>
+       </property>
+      </widget>
+     </item>
      <item>
       <widget class="QPushButton" name="deleteButton">
        <property name="toolTip">