Return the value of an X Windows selection.
The argument TYPE (default `PRIMARY') says which selection,
and the argument DATA-TYPE (default `STRING') says
how to convert the data.
TYPE may be any symbol (but nil stands for `PRIMARY'). However,
only a few symbols are commonly used. They conventionally have
all upper-case names. The most often used ones, in addition to
`PRIMARY', are `SECONDARY' and `CLIPBOARD'.
DATA-TYPE is usually `STRING', but can also be one of the symbols
in `selection-converter-alist', which see.