-The DockState can use the method on the tree and does not need
to reach into the fields of a tree

- These operations are only possible on a binary tree and will
have to have access to the tree going forwards

- Nodes are still stored in a Vec but the position and order of
the nodes no longer follows that of a binary tree.
A node still only allows for a left and right node. So in practice
it is still technically a binary tree. The implementation however
no longer relies on this fact and should make further changes
easer.

- A node can no longer be split on its own.

* Fix crash after calling `DockState::remove_tab`. (Adanos020#208)

* Remove surface after removing the last remaining tab, if the surface isn't `Main`.

* Add explicit panic in `Tree::remove_leaf` if the `Tree` is empty.

* Bump patch version, update changelog.

* Include instructions on how to run examples in Readme. (Adanos020#209)

* Fix keyboard focus not working inside `DockArea`s

* Add visual indicators when tab buttons are focused

* Allow current tab to be switched using the keyboard

* Remove extra focusable areas

I removed focus from two widgets that are not supposed to be focusable.
Also, I added highlighting to the separators when they are focused
because apparently I can't remove the ability to focus them without
breaking the ability to drag the separators.

* Separators can now be moved with the arrow keys

* Add highlighting to active tabs that are keyboard focused

* Update changelog

* Add tab styles for keyboard-focused tabs

* Don't move separators unless Ctrl or Shift is down

This is so that using the arrow keys to change the keyboard focus will
still work normally.

* Clippy

---------

Co-authored-by: Adam Gąsior <[email protected]>

* Fix crash after calling `DockState::remove_tab`. (Adanos020#208)

* Remove surface after removing the last remaining tab, if the surface isn't `Main`.

* Add explicit panic in `Tree::remove_leaf` if the `Tree` is empty.

* Bump patch version, update changelog.

* Include instructions on how to run examples in Readme. (Adanos020#209)

* Fix keyboard focus not working inside `DockArea`s

* Add visual indicators when tab buttons are focused

* Allow current tab to be switched using the keyboard

* Remove extra focusable areas

I removed focus from two widgets that are not supposed to be focusable.
Also, I added highlighting to the separators when they are focused
because apparently I can't remove the ability to focus them without
breaking the ability to drag the separators.

* Separators can now be moved with the arrow keys

* Add highlighting to active tabs that are keyboard focused

* Update changelog

* Add tab styles for keyboard-focused tabs

* Don't move separators unless Ctrl or Shift is down

This is so that using the arrow keys to change the keyboard focus will
still work normally.

* Clippy

---------

Co-authored-by: Adam Gąsior <[email protected]>

- When a node is removed this might cause a sibling of the node
to be moved. If the sibling is focused this caused the focused node
to point to an stale node.

- Make the breadth first index iterator public

…binary_tree

- The exact behavior of these functions still need to be discussed.