CodePaster: Register type safe service object

Use Q_DECLARE_INTERFACE et al.
This provides a way to have the dependency on code pasting
optional in the using plugins (VCS, diff editor), while
still being able to use a nice API to perform the pasting itself.

Change-Id: Ia61e0066d552e45031f4aa7fd1f6693b68f92384
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/plugins/diffeditor/diffeditor.qbs

@@ -9,6 +9,10 @@ QtcPlugin {
     Depends { name: "Core" }
     Depends { name: "TextEditor" }
 
+    pluginRecommends: [
+        "CodePaster"
+    ]
+
     files: [
         "diffeditor.cpp",
         "diffeditor.h",