XUtils: move implementation of String_contains_i out of header file

The function strcasestr(3) is only available if _GNU_SOURCE is defined.
If any file includes <string.h> before declaring _GNU_SOURCE, e.g by
including "config.h", compilation fails with the following error:

    In file included from ColumnsPanel.c:8:
    In file included from ./ColumnsPanel.h:12:
    In file included from ./Panel.h:13:
    In file included from ./CRT.h:16:
    In file included from ./Settings.h:17:
    In file included from ./Process.h:15:
    In file included from ./Object.h:17:
    ./XUtils.h:42:11: error: implicit declaration of function 'strcasestr' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
       return strcasestr(s1, s2) != NULL;
              ^
    ./XUtils.h:42:11: note: did you mean 'strcasecmp'?
    /usr/include/strings.h:116:12: note: 'strcasecmp' declared here
    extern int strcasecmp (const char *__s1, const char *__s2)
               ^

Move the implementation to avoid unnecessary includes.
Since LTO is quite common and stable performance should not be impacted
if used.

XUtils.c

@@ -78,6 +78,10 @@ void* xReallocArray(void* ptr, size_t nmemb, size_t size) {
    return xRealloc(ptr, nmemb * size);
 }
 
+inline bool String_contains_i(const char* s1, const char* s2) {
+   return strcasestr(s1, s2) != NULL;
+}
+
 char* String_cat(const char* s1, const char* s2) {
    const size_t l1 = strlen(s1);
    const size_t l2 = strlen(s2);