[Qt] rename rpcconsole window

- rework window title to not include Bitcoin - in front, as no other
  dialog does this
- favor a connect() call over an own function for clearing the traffic
  graph
- write monospace lowercase (seems to be correct after some web search)
  and add a comment that we should avoid / remove fixed font sizes

src/qt/forms/rpcconsole.ui

@@ -11,7 +11,7 @@
    </rect>
   </property>
   <property name="windowTitle">
-   <string>Bitcoin - Debug window</string>
+   <string>Debug window</string>
   </property>
   <layout class="QVBoxLayout" name="verticalLayout_2">
    <item>