[libcalamares] Remove confusing name-qualifiers

- Using Branding::ImageEntry, when ImageEntry is an enum class
   defined *in* Branding, is superfluous, and it also confuses
   moc; the enum type isn't recognized from QML.

src/libcalamaresui/Branding.h

@@ -183,9 +183,9 @@ public:
     void setGlobals( GlobalStorage* globalStorage ) const;
 
 public slots:
-    QString string( Branding::StringEntry stringEntry ) const;
-    QString styleString( Branding::StyleEntry styleEntry ) const;
-    QString imagePath( Branding::ImageEntry imageEntry ) const;
+    QString string( StringEntry stringEntry ) const;
+    QString styleString( StyleEntry styleEntry ) const;
+    QString imagePath( ImageEntry imageEntry ) const;
 
 private:
     static Branding* s_instance;