* plugins/html/plugin.xml.in: Add xhtml_range saver. Used when
exporting to clipboard.

* plugins/html/html.[ch] (xhtml_range_file_save): New function. Save
range as xhtml. Used when exporting to clipboard.

* plugins/html/html.c (write_sheet): Add save_scope parameter. Don't add
caption if scope is 'range'.
(html_file_save): Call write_sheet with save_scope argument.

* src/plugin-service.c (plugin_service_file_saver_read_xml):
Recognize save_scope value "range".

* src/gui-file.c (make_format_chooser): Leave savers with scope
'range' out from menu.

* src/gui-clipboard.c (table_cellregion_write): New
function. Convert to memory buffer in format wanted by clipboard
by pasting into a temporary workbook, saving that to a memory
output stream.
(x_clipboard_get_cb): Export to clipboard as xhtml.

* src/file.h (enum FileSaveScope): Add FILE_SAVE_RANGE, used when
exporting to clipboard.