[AdgToyText] Allow to specify the font style
By default any AdgToyText entity defaults to the current font style
(the ADG_FONT_STYLE_TEXT AdgFontStyleId). Added a property
("font-style") to allow a different _structural_ style and use the
recently introduced API to apply this font style before rendering.
Also added a new API (adg_toy_text_get_extents()) to know the boundary
box of this toy text as returned by cairo_glyph_extents(). The result is
cached together with the glyps and this API will be removed once the
boundary feature will be implemented.