Split trait Layout into Tile, Layout; impl Layout for Text

[dhardy]

pub trait Tile is now the dyn-safe (no assoc. type Data or Sized restriction) super of trait Widget.

trait Tile: Layout with:

pub trait Layout {
    fn size_rules(&mut self, sizer: SizeCx, axis: AxisInfo) -> SizeRules;

    fn set_rect(&mut self, cx: &mut ConfigCx, rect: Rect, hints: AlignHints);

    fn try_probe(&mut self, coord: Coord) -> Option<Id> {
        None
    }

    fn draw(&mut self, draw: DrawCx);

where Layout is now implementable by non-widgets.

fn probe is moved to trait Events since it doesn't need to appear in the dyn Widget vtable and is not applicable to non-widgets`.

Implement Layout for kas::theme::Text.

Allow usage of layout = self.text where text is a Text field.