title: In WiX using edit control, How do I make sure the content is not empty
body: <p>I have the 2 edit control. How do I validate for non empty string.
  Below is the wix file I use. I would like PushButton to be disabled until there are values for WIXUI_AGENTIP and WIXUI_AGENTPORT.</p>
  
  <pre><code>&lt;Dialog Id="MyInstallDirDlg" Width="370" Height="270" Title="!(loc.InstallDirDlg_Title)"&gt;
  &lt;Control Id="Next" Type="PushButton" X="236" Y="243" Width="56" Height="17" Default="yes" Text="!(loc.WixUINext)" /&gt;
  &lt;Control Id="Back" Type="PushButton" X="180" Y="243" Width="56" Height="17" Text="!(loc.WixUIBack)" /&gt;
  &lt;Control Id="Cancel" Type="PushButton" X="304" Y="243" Width="56" Height="17" Cancel="yes" Text="!(loc.WixUICancel)"&gt;
  &lt;Publish Event="SpawnDialog" Value="CancelDlg"&gt;1&lt;/Publish&gt;
  &lt;/Control&gt;
  
  &lt;Control Id="Description" Type="Text" X="25" Y="23" Width="280" Height="15" Transparent="yes" NoPrefix="yes" Text="!(loc.InstallDirDlgDescription)" /&gt;
  &lt;Control Id="Title" Type="Text" X="15" Y="6" Width="200" Height="15" Transparent="yes" NoPrefix="yes" Text="!(loc.InstallDirDlgTitle)" /&gt;
  &lt;Control Id="BannerBitmap" Type="Bitmap" X="0" Y="0" Width="370" Height="44" TabSkip="no" Text="!(loc.InstallDirDlgBannerBitmap)" /&gt;
  &lt;Control Id="BannerLine" Type="Line" X="0" Y="44" Width="370" Height="0" /&gt;
  &lt;Control Id="BottomLine" Type="Line" X="0" Y="234" Width="370" Height="0" /&gt;
  
  &lt;Control Id="FolderLabel" Type="Text" X="20" Y="60" Width="290" Height="15" NoPrefix="yes" Text="!(loc.InstallDirDlgFolderLabel)" /&gt;
  &lt;Control Id="Folder" Type="PathEdit" X="20" Y="80" Width="320" Height="18" Property="WIXUI_INSTALLDIR" Indirect="yes" /&gt;
  &lt;Control Id="ChangeFolder" Type="PushButton" X="20" Y="100" Width="56" Height="17" Text="!(loc.InstallDirDlgChange)" /&gt;
  
  &lt;Control Id="IPAddressLabel" Type="Text" X="20" Y="130" Width="200" Height="15" TabSkip="no" Text="HostName/IP Address of Server:" /&gt;
  &lt;Control Id="IPAddressEdit" Type="Edit" X="20" Y="140" Width="220" Height="18" Property="WIXUI_AGENTIP" Text="Enter HostName/IP Address Here" /&gt;
                  &lt;!-- port --&gt;
  &lt;Control Id="SQLPortLabel" Type="Text" X="20" Y="160" Width="160" Height="15" TabSkip="no" Text="Port for Sentinel Syslog Server:" /&gt;
  &lt;Control Id="DomainPoolEdit" Type="Edit" X="20" Y="170" Width="220" Height="18" Property="WIXUI_AGENTPORT" Text="Port Number Here" /&gt;               
              &lt;/Dialog&gt;
  </code></pre>