Add an insert button that shows a list of registered blocks, and adds the selected block after the currently focused block.

Requirements for alpha:

• The inserter button will be in the toolbar as the design shows (depends on Toolbar #55).
• The block picker should show the right icon for each block. (done in Inserter - block icons #129)
• After the block is inserted, switch focus to the newly inserted block.

Tasks

Integration with toolbar (2 weeks)

• add new + adder control placeholder (to be connected to the toolbar)
• extract and define adder <-> toolbar interface
• actually connect with toolbar

Other (1 week)

• UI pass (make it not only work but look as per the designs) -> Inserter UI #349 (Beta)