Fix Menu bubble position

[Chrissi2812]

Bug

EditorView.cordsAtPos returns wrong cordinates if we are at a linebreak.

Fixed

This will fix #215

[pangrr]

Anyone can review this? It's really helpful.

[Chrissi2812]

As mentioned by @bvkimball in #239 the menuBubble was sometimes placed outside the editor bounding box.

When enabled via:

<editor-menu-bubble :editor="editor" :keepInBounds="true">

It will keep the menuBubble inside it's offsetParent element.

Currently the keepInBounds option is NOT reactive.
This will be part of another PR because this involved a rewrite of the Plugin to an Extension so it's handled by the ExtensionManager.

[philippkuehn]

Thanks for that PR, Chris! I'm not sure if it's a good idea to overwrite core functions of ProseMirror but I will merge that for now.