Minor improvements in Plot Dock columns

The Column Field is resized to the content.

The dummy column has been removed and the header no longer stretches the
last section property.

The table has alternating row colors for easy following of the field
checks.

src/PlotDock.cpp

@@ -117,6 +117,7 @@ void PlotDock::updatePlot(SqliteTableModel* model, BrowseDataTableSettings* sett
                         columnitem->setCheckState(PlotColumnX, Qt::Unchecked);
                 }
             }
+            ui->treePlotColumns->resizeColumnToContents(PlotColumnField);
 
             // Add a row number column at the beginning of the column list, but only when there were (other) columns added
             if(ui->treePlotColumns->topLevelItemCount())

src/PlotDock.h

@@ -57,7 +57,6 @@ private:
         PlotColumnField = 0,
         PlotColumnX = 1,
         PlotColumnY = 2,
-        PlotColumnDummy = 3
     };
 
     Ui::PlotDock* ui;

src/PlotDock.ui

@@ -26,12 +26,18 @@
         <verstretch>2</verstretch>
        </sizepolicy>
       </property>
+      <property name="alternatingRowColors">
+       <bool>true</bool>
+      </property>
       <property name="columnCount">
-       <number>4</number>
+       <number>3</number>
       </property>
       <attribute name="headerDefaultSectionSize">
        <number>100</number>
       </attribute>
+      <attribute name="headerStretchLastSection">
+       <bool>false</bool>
+      </attribute>
       <column>
        <property name="text">
         <string>Columns</string>
@@ -47,20 +53,6 @@
         <string>Y</string>
        </property>
       </column>
-      <column>
-       <property name="text">
-        <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>
      <widget class="QCustomPlot" name="plotWidget" native="true">
       <property name="sizePolicy">