GUI2/Canvas: consolidate set_width and set_height into a single function

This is to enable me to do some testing with high DPI support.
Also there's never a situation where you'd only want to set one of these.

src/gui/core/canvas.cpp

@@ -1578,6 +1578,14 @@ void canvas::clear_shapes(const bool force)
 	drawn_shapes_.erase(iter, drawn_shapes_.end());
 }
 
+void canvas::set_size(unsigned w, unsigned h)
+{
+	update_size(w_, w);
+	update_size(h_, h);
+
+	invalidate_cache();
+}
+
 void canvas::update_size(unsigned int& value, unsigned int new_value)
 {
 	if(value != new_value) {

src/gui/core/canvas.hpp

@@ -144,23 +144,13 @@ public:
 
 	/***** ***** ***** setters / getters for members ***** ****** *****/
 
-	void set_width(const unsigned width)
-	{
-		update_size(w_, width);
-		invalidate_cache();
-	}
+	void set_size(unsigned w, unsigned h);
 
 	unsigned get_width() const
 	{
 		return w_;
 	}
 
-	void set_height(const unsigned height)
-	{
-		update_size(h_, height);
-		invalidate_cache();
-	}
-
 	unsigned get_height() const
 	{
 		return h_;

src/gui/widgets/styled_widget.cpp

@@ -267,8 +267,7 @@ void styled_widget::place(const point& origin, const point& size)
 	// resize canvasses
 	for(auto & canvas : canvases_)
 	{
-		canvas.set_width(size.x);
-		canvas.set_height(size.y);
+		canvas.set_size(size.x, size.y);
 	}
 
 	// Note we assume that the best size has been queried but otherwise it