Support for Sparkle criticalUpdate

Currently, there is a minimal support for criticalUpdate tag in appcast feed.

<sparkle:tags>
    <sparkle:criticalupdate/>
</sparkle:tags>

Note: This does not parse the specific version string embedded in the criticalUpdate tag. It also doesn't deal with the case where criticalUpdate tag appears outside of sparkle:tags. It can be extended to support the way sparkle 2 does with a bit of effort.

On detecting this particular tag in the appcast.xml, the installer wizard will make the following changes to Update Available Dialog State

• Hide the skip & remind me later buttons.
• Hide the close button on the title bar of the dialog box

PR: https://github.com/vslavik/winsparkle/pull/244/files