Use core layouts for placing blocks

Problem/Motivation

At the moment there are only 4 hardcoded variants of how the blocks can be placed into dashboard: 1, 2, 3, 4 columns. It would be more convenient to use the layouts instead and allow users to create their own ways of placing the blocks with flexible columns/grids/bricks, etc.

Proposed resolution

Add layout discovery support to the module. Convert columns setting to layout select. Convert current 1, 2, 3, 4 columns display options to layouts.

Remaining tasks

Implement proposed solution.

User interface changes

User will have a layout select instead of just columns. It should be still possible to move block items among the layout regions.

API changes

None

Data model changes

New property for layout selection.