Refine configuration bar

[danielmolnar]

This PR makes the following changes:

1. Adjusts the layout of the "null" button in the configuration bar to make it look more aligned and also less prone to overflow for longer widget names

2. Adds a border around the color picker widget for when its color is transparent

3. Adds "cancel" action to the color picker which resets the color to its initial value

   

[robiness]

Nice polishing! 👌