Add `Tree::set_width` and `set_height` functions

[rafaga]

I'm trying to make an application where I want the tree don't get all the available space on CentralPanel, Added two new functions to limit the space that takes Tree on the parent container, so you can put widgets outside the tree without using SidePanel or Botton/top Panels.

the proposed functions are

 pub fn set_height(&mut self, height: f32) 

 pub fn set_width(&mut self, width: f32)

this also resolves #69 because uses ui.allocate_space(...); with the resolved rect

[emilk]

needs a merge fix