CppIncludesFilter: Change default shortcut from 'a' to "ai"

Reason: 'a' is also used for "Files in Any Project",
so there was no way of using both filters exclusively.

Task-number: QTCREATORBUG-19531
Change-Id: Icba0e993b004eb72b8cc419f52b171c5289bc0a1
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cpptools/cppincludesfilter.cpp

@@ -132,7 +132,7 @@ CppIncludesFilter::CppIncludesFilter()
 {
     setId("All Included C/C++ Files");
     setDisplayName(tr("All Included C/C++ Files"));
-    setShortcutString("a");
+    setShortcutString("ai");
     setIncludedByDefault(true);
     setPriority(ILocatorFilter::Low);