Change 'Clear' button string to 'Reset'

Easier to understand what the button does (it resets the graph view).

'Clear' might mean that the graph is emptied and stops updating, whereas
its easier to see that you're just starting fresh with 'Reset'.

src/qt/forms/debugwindow.ui

@@ -645,7 +645,7 @@
            <item>
             <widget class="QPushButton" name="btnClearTrafficGraph">
              <property name="text">
-              <string>&amp;Clear</string>
+              <string>&amp;Reset</string>
              </property>
              <property name="autoDefault">
               <bool>false</bool>