ExtensionManager: Introduce extensions service response parser and model

This adds a parser for the JSON response of the extension rest API. The
data, combined with the PluginSpecs of local plugins, serve as data
model for the extension mode view.

A couple of "packs" are provided as test data.

Change-Id: I5ce961a9de9bf54ca745e5e5a5e584b1698e6ac6
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/extensionmanager/CMakeLists.txt

@@ -9,4 +9,14 @@ add_qtc_plugin(ExtensionManager
     extensionmanagerwidget.h
     extensionsbrowser.cpp
     extensionsbrowser.h
+    extensionsmodel.cpp
+    extensionsmodel.h
+)
+
+extend_qtc_plugin(ExtensionManager
+  CONDITION WITH_TESTS
+  SOURCES
+    extensionmanager_test.cpp
+    extensionmanager_test.h
+    extensionmanager_test.qrc
 )