Use text/uri-list instead of x-moz-url in wxGTK wxURLDataObject.

Use text/uri-list instead of x-moz-url in wxGTK wxURLDataObject.

The standard exchange format for URLs is text/uri-list and not the deprecated
and Firefox-specific x-moz-url, support for which was moreover implemented
incorrectly anyhow.

Also add an example of copying URLs to the dnd sample.

See https://developer.mozilla.org/En/DragDrop/Recommended_Drag_Types for more
information.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72159 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775