Range of plot points can be selected in graph and in table

The user is now able of selecting a range of contiguous points in any of
the graphs of the plot, through rectangle selection or click in single
point. This selection triggers automatically the selection of the
corresponding lines in the browsed table. In this way the user can analyse
the plot in detail.

Two other minor improvements: spelling error fixed and the dummy column in
the plot selection table has now transparent foreground (intends to avoid
wondering of the user about what functionality provides this column).

src/PlotDock.ui

@@ -6,7 +6,7 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>478</width>
+    <width>515</width>
     <height>553</height>
    </rect>
   </property>
@@ -49,7 +49,16 @@
       </column>
       <column>
        <property name="text">
-        <string>_</string>
+        <string notr="true">_</string>
+       </property>
+       <property name="foreground">
+        <brush brushstyle="NoBrush">
+         <color alpha="0">
+          <red>0</red>
+          <green>0</green>
+          <blue>0</blue>
+         </color>
+        </brush>
        </property>
       </column>
      </widget>