Plugins: Add skeleton for new ExtensionManager plugin

This initial commit is merely a mockup of a ExtensionManager concept.

Change-Id: I19b2285667678a86097c043cc27a554545559eff
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/extensionmanager/CMakeLists.txt

@@ -0,0 +1,12 @@
+add_qtc_plugin(ExtensionManager
+  PLUGIN_DEPENDS Core
+  SOURCES
+    extensionmanager.qrc
+    extensionmanagerconstants.h
+    extensionmanagerplugin.cpp
+    extensionmanagertr.h
+    extensionmanagerwidget.cpp
+    extensionmanagerwidget.h
+    extensionsbrowser.cpp
+    extensionsbrowser.h
+)