Fix size-request that rely on Pango::Layout

A Pango::Layout created by Gtk::Widget::get_pango_context ()
does not have any font set. The Font is inherited from the
layout's context.

The actual font is set when the layout is used in
on_expose_event() via get_window ()->create_cairo_context ().

In some cases we rely on the font-size to calculate the widget's
size before rendering. At this point in time there is no drawing
context from which to inherit the style.

Furthermore, even after a call to `ensure_style()` in the c'tor
of a Widget get_font() simply returns the default value.

All Widgets that rely an Pango::Layout font size during
size-requests have to explicitly set the font.

This fixes various overlarge buttons with the initial default layout.

libs/widgets/widgets/ardour_button.h

@@ -165,9 +165,11 @@ class LIBWIDGETS_API ArdourButton : public CairoWidget , public Gtkmm2ext::Activ
 
 	void set_text_internal ();
 	void recalc_char_pixel_geometry ();
+
 	unsigned int _char_pixel_width;
 	unsigned int _char_pixel_height;
-	float _char_avg_pixel_width;
+	float        _char_avg_pixel_width;
+	bool         _custom_font_set;
 
 	int   _text_width;
 	int   _text_height;