Utilise the "Add Block" popover on the add button & other controls

[ScopeyNZ]

Given #338 adds a popover with a list of widgets that can be added - the existing "Add" button will need to be replace to use this.

Additionally the popover needs to show when adding a block "in-line" as per the designs.

A/Cs

• The "add block" popover shows when you click on the "Add" button at the top of an elemental area
• After clicking this "Add" button, the popover should be the next item in the tabindex.
• A small blue line with a (+) icon shows when hovering near/between blocks
• Clicking on the aformentioned line will open the "add block" popover

[raissanorth]

With the latest commit of https://github.com/dnadesign/silverstripe-elemental/pull/421/commits all ACs are fulfilled. Related task, if outstanding, are captured in new tickets.