New helper function, GetFileMask - This returns a filem ask for OpenDial...

...og components.  The idea is that it returns all of the file types we support in some form.  This should help with Demo programs.

Disable HTML file parsing in Indy.  That can change the charset and MIME type in THtmlViewer based on meta tags and that's not desirable as THtmlViewer does it's own handling.
For XHTML parsing, attempt to extract the charset from the XML declaration.
Added comment about charset priority in HTML and XHTML.
THtmlParser now has an IsXHTML property that can be set by the THTMLViewer for XHTML files.  This is so that XHTML can be parsed differently than regular HTML.  There are some subtle but potentially important differences between the two types.