Added gtk_cell_renderer_get_aligned_area() and class vfunc.

Added gtk_cell_renderer_get_aligned_area() and class vfunc.

Since a cell renderer might use more space than the natural
size when recieving expand space it's impossible to know how
much space is actually used to render content.

Adding this virtual method to allow text renderers to implement
it, the base default method uses height-for-width apis and aligns
the cell assuming the renderer uses a fixed size.

This commit removes the similar code from gtkcellarea and
subclasses.