Renamed layoutToolbar.html.twig to more accurately describe its purpose.
BlockManager and BlockHandler services can now have a PageManager service object attached to them.

BlockHandler get_handler() method refactored to getHandler() for naming scheme compliance.

Also other code fixes. Probably.
layoutToolbar.html.twig removal commit (from rename in older commit)
Bootstrap Modal element style improvements.
BlockCollection added to CMSPageCore.blocks. 
 - Collection is populated as blocks are accessed. 
 - CMSPageCore.populateInitialModels() will add all page blocks to the BlockCollection.