Fixed regression from 4139b43cc9 (fixes #4215)

This fixes the buttons in the editor's terrain palette being unselectable. Since the gui::widget class
had a virtual base class, the class was not trivially copy-constructable. We need to explicitly define
the copy constructor so we can initialize the sdl_handler base class.

src/widgets/widget.cpp

@@ -30,6 +30,27 @@ namespace gui {
 
 bool widget::mouse_lock_ = false;
 
+widget::widget(const widget& o)
+	: events::sdl_handler()
+	, focus_(o.focus_)
+	, video_(o.video_)
+	, restorer_(o.restorer_)
+	, rect_(o.rect_)
+	, needs_restore_(o.needs_restore_)
+	, state_(o.state_)
+	, hidden_override_(o.hidden_override_)
+	, enabled_(o.enabled_)
+	, clip_(o.clip_)
+	, clip_rect_(o.clip_rect_)
+	, volatile_(o.volatile_)
+	, help_text_(o.help_text_)
+	, tooltip_text_(o.tooltip_text_)
+	, help_string_(o.help_string_)
+	, id_(o.id_)
+	, mouse_lock_local_(o.mouse_lock_local_)
+{
+}
+
 widget::widget(CVideo& video, const bool auto_join)
 	: events::sdl_handler(auto_join), focus_(true), video_(&video), rect_(EmptyRect), needs_restore_(false),
 	  state_(UNINIT), hidden_override_(false), enabled_(true), clip_(false),

src/widgets/widget.hpp

@@ -69,6 +69,7 @@ public:
 	virtual void process_tooltip_string(int mousex, int mousey);
 
 protected:
+	widget(const widget& o);
 	widget(CVideo& video, const bool auto_join=true);
 	virtual ~widget();