xref: Input history and minor tweaks

xref: Input history and minor tweaks

* lisp/progmodes/xref.el (xref--current): Rename from `xref--selected'.
(xref--inhibit-mark-current): Rename from
`xref--inhibit-mark-selected'.  Update the usages.
(xref-quit): Reword the docstring.  Kill buffers after quitting
windows instead of before.
(xref--insert-xrefs): Tweak help-echo.
(xref--read-identifier-history, xref--read-pattern-history):
New variables.
(xref--read-identifier, xref-find-apropos): Use them.