Bar charts and Axis Type column in Plot Dock

String type columns are now selectable as X axis. In that case a bar chart
is plotted where the column values are used as bar labels.

A new Axis Type column is added to the axis selection table, so the user
knows before-hand how the plot will be drawn.

Line type and point shape combo-boxes are not enabled for bar charts, since
they don't make sense.

Columns that make no sense for the Y axis as strings and date/times are not
selectable for that axis.

src/PlotDock.ui

@@ -30,7 +30,7 @@
        <bool>true</bool>
       </property>
       <property name="columnCount">
-       <number>3</number>
+       <number>4</number>
       </property>
       <attribute name="headerDefaultSectionSize">
        <number>100</number>
@@ -53,6 +53,11 @@
         <string>Y</string>
        </property>
       </column>
+      <column>
+       <property name="text">
+        <string>Axis Type</string>
+       </property>
+      </column>
      </widget>
      <widget class="QCustomPlot" name="plotWidget" native="true">
       <property name="sizePolicy">