Remove Markdown from DESCRIPTION and add HTML support with X-ALT-DESC

As has been pointed out including markdown tags in the DESCRIPTION breaks other applications displaying VJOURNAL or VTODO events.

Remove the (limited) support for markdown that exists and work towards allowing HTML to be included in the description using the X-ALT-DESC element in parallel with DESCRIPTION

One problem is likely to be the potential for a disconnect between the text of DESCRIPTION element and the text of the X-ALT-DESC if the entry is edited by an application that doesn't use X-ALT-DESC and then sync'd back into jtxBoard.

This would require code to check if the LAST-MOD date had changed and if it had merge the text with changes into the html version - potentially quite a complex thing to do - strip tags from html version, strip newlines from the new plain text version, get the differences, do a search and replace for each difference on the html version and save it. If X-ALT-DESC was missing on imported/sync'd entries then it would be generated from DESCRIPTION by simply replacing newlines with paragraphs.

The html version X-ALT-DESC would be used for viewing in jtxBoard and as the master for editing and the plain text DESCRIPTION version would be generated on save by replacing </p> and </h[n]> tags with \n\n and <br /> and some others with \n, plus handling lists etc sensibly. You could limit the html tags that would be supported to make life easier.

In the first instance the simple text editor could be used with html entered manually, but ideally some form of wysiwyg editor component would be needed with a simple toolbar - does such a thing like TinyMCE exist for Android? I've never coded for android - just php and js nowadays, so if what I am suggesting is unfeasible please say and close this issue :-)