Append Cart block and add modal buttons

Thanks, looks good!

Couple of things we should consider:

1. The 'cart' id used to append if you're adding the first item is configured when creating the block, 'cart' is the default for the first block but depending on the settings it might not be the block that should be appended, or the user might have named it something different entirely.
2. I wonder if we could programmatically determine the id of the block's region and append the block to that ID so we don't have to ask for it?
3. Finally, I wonder how we can programmatically determine which blocks are already rendered and which aren't. Perhaps based on the block type or block settings we can tell which cart blocks would be hidden if there were 0 items in the cart and append those instead of replacing.

I added a Keep Browsing button and reconfigured the settings form a little bit. I also made the dialog window responsive.

Thanks kamkejj & amklose! These changes were eventually rolled into a larger changeset and committed back to the module, so I gave you guys credit for the commit as well.