bug#28439: suggestion: support case-independent xref-find-definitions prompt TAB

[Winston]

xref-find-definitions UI suggestion:

   Since xref-find-definitions is willing to match names typed in all
   lower case to mixed case names, TAB (show alternatives) should, too.
   If there's not already a way to make TAB do so, then a way should be
   added.

Discussion:

   Suppose I have Foo1() and Foo2().  Entering "foo1" or "foo2" at the
   prompt works if the name is complete and unique, even though I used
   all lower case.  TAB, however will only show alternatives if I've
   typed "Foo" TAB, while "foo" TAB will say "No matches".

   While it's obvious why the code works that way, I claim users are
   going to expect TAB to offer completions for any string with which
   xref-find-definitions would succeed, and thus that TAB should be
   capable of presenting a less case-dependent alternatives list.

   I was surprised when it didn't.  I had typed in enough of a tag name
(in all lower case) to match uniquely, and then typed TAB expecting the
single completion (perhaps in the prompt line itself rather than a
separate window, possibly with the string I had entered being converted
to its correct mixed case), and was surprised when it said No match.

   This is also another difference compared to find-tag, where one only
needs to uniquely partial match a name to get to it.  With
xref-find-definitions, I can't go to the partial name and TAB doesn't
complete the name even when there's only one match (because the typed
string had the wrong case), so it takes more typing (or correct use of
the shift key) than find-tag did.
 -WBE