New way to open debug consoles

Replace broken old utils.debug(...) (since the utils.js → Utils.qml
transition) with a declarative approach:
a console can now be opened with by placing a `DebugConsoleLoader {}`
anywhere.
mainUI has a default console opened by F1/alt-shit-c when no other
console is loaded.

src/gui/UI.qml

@@ -35,7 +35,15 @@ Item {
         }
     }
 
-    GlobalShortcuts { id: shortcuts }
+    GlobalShortcuts {
+        id: shortcuts
+        defaultDebugConsoleLoader: debugConsoleLoader
+    }
+
+    DebugConsoleLoader {
+        id: debugConsoleLoader
+        active: false
+    }
 
     HImage {
         id: mainUIBackground