Rebrand to Bitcoin Core

Only messages for now, executable names and other file names
can be changed later if necessary and safe.

Do not do an all-sweeping change. Some occurences of Bitcoin-Qt need to
be kept:

- Applicationname: this is used to determine the registry entry names,
  we don't want to lose settings over a silly name change.
- Where it refers to the executable name instead of the product name.

src/qt/forms/aboutdialog.ui

@@ -11,7 +11,7 @@
    </rect>
   </property>
   <property name="windowTitle">
-   <string>About Bitcoin</string>
+   <string>About Bitcoin Core</string>
   </property>
   <layout class="QHBoxLayout" name="horizontalLayout_2">
    <item>
@@ -50,7 +50,7 @@
           <cursorShape>IBeamCursor</cursorShape>
          </property>
          <property name="text">
-          <string>&lt;b&gt;Bitcoin&lt;/b&gt; version</string>
+          <string>&lt;b&gt;Bitcoin Core&lt;/b&gt; version</string>
          </property>
          <property name="textInteractionFlags">
           <set>Qt::LinksAccessibleByMouse|Qt::TextSelectableByKeyboard|Qt::TextSelectableByMouse</set>
@@ -91,7 +91,7 @@
         <cursorShape>IBeamCursor</cursorShape>
        </property>
        <property name="text">
-        <string notr="true">Copyright &amp;copy; 2009-YYYY The Bitcoin developers</string>
+        <string notr="true">Copyright &amp;copy; 2009-YYYY The Bitcoin Core developers</string>
        </property>
        <property name="textFormat">
         <enum>Qt::RichText</enum>