mono revamp drag-n-drop and clipboard support for X11 / linux

moves the content handling from X11Dnd to the new X11SelectionHandler
re-writes the X11 -> mono content detection to deal with mime-type parameters
re-writes the mono -> X11 content detection to deal with native types, URI, IEnumerable<Uri> and null-objects
expands DataFormats.StringFormat/Text/UnicodeText to properly handle charset encodings support text/plain, UTF8_STRING, text/unicode, TEXT as well as STRING
adds DataFormats.DataFormats.Bitmap support for image/png, image/bmp, image/jpeg, image/gif and image/tiff
adds DataFormats.FileDrop support for FILE_NAME, text/x-moz-url and _NETSCAPE_URL
adds DataFormats.CommaSeparatedValue support for text/csv
DataFormats.Serializable now uses a GUID header bevore the actual payload

regarding 1) X11's clipboard and DND content handling is identical and differs only in the selection's ID used thus no need to implement it twice lets find all potential issues in the less critical DND-functions first and then later switch X11Clipboard's content handling to X11SelectionHandler

regarding 2) for example to deal with "text/plain;charset=utf-8"

regarding 8) in line with Window implementation