ColumnCellCheckBox: Increase default size to 16 pixels

With 13 pixels the checkbox would get clipped, and 16 is the default
size of the Gtk.CheckButton widget.
CellContext: Add a StyleContext property

This is just a getter, a short-hand for Widget.StyleContext.

We've been inconsistently using either Widget.StyleContext or
Theme.Widget.StyleContext, so this new property should make things
clearer.
Hyena.Gui: Clean up references to StyleContext

In ColumnCell subclasses, use the new CellContext.StyleContext property.

In the ListView widget itself, we don't need to go through the
CellContext, as the widget set in the CellContext is always the ListView
itself.
ColumnCellText: Set the correct StateFlags on the StyleContext

This ensures that the text is in the right color when the cell is
selected.

This fixes a regression introduced in commit 4f76fab0, which
causes the cell state to be ignored when drawing with RenderLayout.