Enforced bounds on `create_widget` and `VecTracker`

Also derived traits on Fragment and VecTracker

[RFC 2] Text Measure

Separated `KeyboardInput` into `KeyUp` and `KeyDown` events

Added a bevy feature to kayak_ui. 

Updated to the 0.6 release of bevy. Fixed some bugs.

TextBox text style improvements

Update bevy

These allow us to pass along metadata with a keyboard event
(specifically here, which modifier keys are active)

* Use colour style for the text
* Use gray colour for the placeholder text
* Add more color constant

Also added the `remove` method

Add: `Fold` widget

Added `use_effect` hook macro