MSI installer: offer to display the README file after install.

This is a thing that the Inno Setup installer did, and that I didn't
get round to replicating when I rushed out the initial MSI in a hurry.

I've checked that this doesn't prevent unattended installation by
administrators: running 'msiexec /q /i putty-whatever.msi' as
administrator still installs silently after this change, without
popping up the README unexpectedly on anyone's desktop as a side
effect.

(I _think_ - but I'm still a long way from an MSI expert - that that's
because /q turns off the whole UI part of the MSI system, and the
loading of README is actually triggered by the transition away from
the final UI dialog box, which we now never visit in the first place.)

Buildscr

@@ -171,7 +171,7 @@ delegate windows
   in putty/doc with htmlhelp do/win hhc putty.hhp & type putty.chm >nul
 
   # Build the WiX MSI installer.
-  in putty/windows with wix do/win candle -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -sval installer.wixobj
+  in putty/windows with wix do/win candle -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj
 
   # Build the old Inno Setup installer.
   in putty/windows with innosetup do/win iscc putty.iss

windows/installer.wxs

@@ -420,6 +420,9 @@
       <Publish Dialog="MaintenanceTypeDlg" Control="RemoveButton" Event="NewDialog" Value="VerifyReadyDlg">1</Publish>
       <Publish Dialog="MaintenanceTypeDlg" Control="Back" Event="NewDialog" Value="MaintenanceWelcomeDlg">1</Publish>
 
+      <Publish Dialog="ExitDialog" Control="Finish" Event="DoAction" 
+               Value="LaunchApplication">WIXUI_EXITDIALOGOPTIONALCHECKBOX = 1 and NOT Installed</Publish>
+
       <!--
           This ARPNOMODIFY flag prohibits changing the set of
           installed features, which would otherwise be possible by
@@ -444,6 +447,16 @@
       <Property Id="ARPNOMODIFY" Value="1" />
     </UI>
 
+    <!--
+        Offer to display README after installation.
+    -->
+    <Property Id="WIXUI_EXITDIALOGOPTIONALCHECKBOXTEXT"
+              Value="View README file" />
+    <Property Id="WIXUI_EXITDIALOGOPTIONALCHECKBOX" Value="1" />
+    <Property Id="WixShellExecTarget" Value="[#README_File]" />
+    <CustomAction Id="LaunchApplication" BinaryKey="WixCA"
+                  DllEntry="WixShellExec" Impersonate="yes" />
+
     <!-- Glue: tell the install dir part of the UI what id my actual
          install dir is known by. Otherwise the former won't know how
          to alter the setting of the latter. -->