table-header: use gtk_widget_create_pango_context() for header buttons
Since we temporarily set custom style classes for the header button on
the table's style context, we cannot rely on the PangoContext used by
gtk_widget_create_pango_layout(), since the font values it will use are
cached by GtkWidget.
By creating a new PangoContext and using that to create our Pango
layout, the text we render will correctly support the properties
specified by the theme (such as bold column-header buttons as specified
by Adwaita).
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