Add button by element

[aminya]

Fixes #291

Motivation

Similar to Status-Bar that has a function that accepts an HTML element, we should allow this too. This feature in combination with the typical addButton allows complex usage of Toolbar.

item = toolbar.addItem({
  element: myElement,
  priority: someNumber // optional
})

Then, the user can do whatever they wanna do with it. For example, define custom buttons like dropdowns, define click callbacks, hide it when they want, etc. It easily solves #43 and #192 by removing the limitations.

Changes

• Defined ToolBarItem which is the parent (super) of all classes (basically refractors the common part of ToolBarButton and ToolBarSpacer)
• addItem function that constructs a ToolBarItem and adds it to the Toolbar.
• Made ToolBarButton inherit from ToolBarItem
• Made ToolBarSpacer inherit from ToolBarItem
• Made classes named exports

[suda]

Nice! LGTM :) If you add some docs to the readme, we could ship it! Do you agree @ericcornelissen ?

[ericcornelissen]

Implementation looks good to me 👍 (not sure if the comment about calling super in the constructor are necessary, as that's pretty common for classes 🤔)

[ericcornelissen]

Small detail, but I would prefer to move the addItem documentation after the addSpacer documentation as addItem is the "most advanced" method (or at least after addButton, in alphabetical order).

[aminya]

@ericcornelissen done in 533d8f6