Rename SearchPath class Searchpath

Windows headers define SearchPath which means we have to undefine it
where necessary. This is a pain and can be tricksy, so I feel renaming
the class slightly is the easiest solution.

libs/pbd/test/test_common.cc

@@ -25,7 +25,7 @@
  * in an installed location on windows or by setting an environment variable
  * on unix.
  */
-PBD::SearchPath
+PBD::Searchpath
 test_search_path ()
 {
 #ifdef PLATFORM_WINDOWS