qmljs: added persistent trie

Add a presistent trie to store imports, and provide a better (IMHO)
completion algorithm.
The trie is quite generic and it might be worth while to move it to utils.

Change-Id: I4081346af6215b1ee8ff14bd063c2a021d7c8218
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/libs/qmljs/qmljs.qbs

@@ -97,6 +97,8 @@ QtcLibrary {
         "parser/qmljsmemorypool_p.h",
         "parser/qmljsparser.cpp",
         "parser/qmljsparser_p.h",
+        "persistenttrie.cpp",
+        "persistenttrie.h",
         "consolemanagerinterface.cpp",
         "consolemanagerinterface.h",
         "consoleitem.cpp",