Use both URL-specific and plain text formats in wxGTK wxURLDataObject.

Use both URL-specific and plain text formats in wxGTK wxURLDataObject.

Just as in wxMSW, it makes sense to put URLs on clipboard (or drag them) in
both URL-specific and plain text formats to facilitate pasting (or dropping)
them into other applications.

So make wxURLDataObject in wxGTK a composite data object containing both its
old data object and wxTextDataObject.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72161 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775