Change highlight region

The ListView style was using "Stretch" for TextBoxes in the code list,
which caused the background of the entire address / label / operand
field to be drawn in the highlight color, rather than just the area
covered by the text.  This is fine for address and label, but it just
felt weird for the operand field because that tends to be very wide
(to accommodate strings, bulk hex data, etc).

There doesn't seem to be a way to specify HorizontalContentAlignment
per-column in WPF.  (Note this is different from HorizontalAlignment,
which *is* is a per-column property.)

This changes the style to use HorizontalContentAlignment=Left, so
the highlight just covers the text.  The only time this causes a
functional change is when you highlight an operand for a line that
doesn't have a label, because instead of highlighting an empty
rectangle you now see nothing at all.  (The address field is still
highlighted though.)

SourceGen/WpfGui/CodeListItemStyle.xaml

@@ -329,7 +329,7 @@ See also https://github.com/fadden/DisasmUiTest
         <!-- This causes individual TextBoxes to stretch to fill the column.  This is nice
         because the cell-highlight stuff will set the background on the full cell rather than
         just the bit with the text. -->
-        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
+        <Setter Property="HorizontalContentAlignment" Value="Left"/>
 
         <Style.Triggers>
             <DataTrigger Binding="{Binding Path=IsLongComment}" Value="True">