qbs build: Introduce new module "qtc".

The qtc module gathers properties that used to live in the top-level
project file. This is the first step towards making it possible to build
plugins against an installed Qt Creator ("out of source build").

Change-Id: Ia1514cc9c888e80be01b308e908de48980fcbdb8
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/plugins/cpptools/cpptools.qbs

@@ -105,7 +105,7 @@ QtcPlugin {
 
     Group {
         name: "Tests"
-        condition: project.testsEnabled
+        condition: qtc.testsEnabled
         files: [
             "cppcodegen_test.cpp",
             "cppcompletion_test.cpp",