Fix unicode strings can't be pasted correctly

Fix unicode strings can't be pasted correctly

* lisp/select.el (x-select-request-type): Use text/plain;charset=utf-8
instead.
(gui--selection-value-internal): same.
(gui-get-selection): Treat text/plain;charset=utf-8 as utf-8.
* src/pgtkselect.c (Fpgtk_get_selection_internal): Support
text/plain;charset=utf-8 as foreign-selection.
(syms_of_pgtkselect): Add new symbol.