Custom statusbar

The statusbar is created since a separate widget class and has three
different labels (settable through properties) for CPU, memory and
number of processes. This makes it more convenient to change one or
another value and it gives a nicer look.

src/process-window.ui

@@ -131,15 +131,7 @@
           </packing>
         </child>
         <child>
-          <object class="GtkStatusbar" id="process-statusbar">
-            <property name="visible">True</property>
-            <property name="spacing">2</property>
-          </object>
-          <packing>
-            <property name="expand">False</property>
-            <property name="pack_type">end</property>
-            <property name="position">2</property>
-          </packing>
+          <placeholder/>
         </child>
       </object>
     </child>