mu4e: rename to mu4e-headers-search-hook

The old name mu4e-headers-search-pre-hook was unnecessarily long.

NEWS.org

@@ -21,11 +21,11 @@
      no exact match; small convenience that affects most the
      single-char option reading in mu4e.
    - Now, just before executing any search, a hook-function
-     `mu4e-headers-search-pre-hook` is invoked, which receives the
+     `mu4e-headers-search-hook` is invoked, which receives the
      search expression as its parameter.
    - In addition there's a `mu4e-headers-search-bookmark-hook` which
      gets called when searches get invoked as a bookmark (note that
-     `mu4e-headers-search-pre-hook` will also be called just
+     `mu4e-headers-search-hook` will also be called just
      afterwards). This hook also receives the search expression as its
      parameter.