Fix: we should include gdl headers in accordance to pkg-config

The pkg-config of GDL advises to include the GDL subdir,
which in turn contains a directory "gdl" with the headers.
This is fine, because it makes this name switch work
painless. Thus, please just #include <gdl/gdl-whatever.h>

src/gui/panels/panel.cpp

@@ -24,7 +24,7 @@
 #include "../gtk-lumiera.hpp"
 #include "../workspace/panel-manager.hpp"
 #include "../workspace/workspace-window.hpp"
-#include <libgdl-1.0/gdl/gdl-dock-item-grip.h>
+#include <gdl/gdl-dock-item-grip.h>
 
 using namespace Gtk;
 

src/gui/panels/panel.hpp

@@ -28,7 +28,7 @@
 #define PANEL_HPP
 
 #include "../gtk-lumiera.hpp"
-#include <libgdl-1.0/gdl/gdl-dock-item.h>
+#include <gdl/gdl-dock-item.h>
 
 #include "../widgets/panel-bar.hpp"
 

src/gui/workspace/panel-manager.hpp

@@ -28,7 +28,7 @@
 #ifndef PANEL_MANAGER_HPP
 #define PANEL_MANAGER_HPP
 
-#include <libgdl-1.0/gdl/gdl.h>
+#include <gdl/gdl.h>
 #include <typeinfo>
 
 #include "../panels/panel.hpp"

src/gui/workspace/workspace-window.cpp

@@ -26,11 +26,11 @@
 #  include <libintl.h>
 #endif
 
-#include <libgdl-1.0/gdl/gdl-tools.h>
-#include <libgdl-1.0/gdl/gdl-dock.h>
-#include <libgdl-1.0/gdl/gdl-dock-item.h>
-#include <libgdl-1.0/gdl/gdl-dock-placeholder.h>
-#include <libgdl-1.0/gdl/gdl-dock-bar.h>
+#include <gdl/gdl-tools.h>
+#include <gdl/gdl-dock.h>
+#include <gdl/gdl-dock-bar.h>
+#include <gdl/gdl-dock-item.h>
+#include <gdl/gdl-dock-placeholder.h>
 
 #include "../gtk-lumiera.hpp"
 #include "workspace-window.hpp"