Mobile: Block toolbar should be responsive

[jasmussen]

Related to #44 and probably best to wait for #660 to be merged: the block toolbar should be responsive.

Part of this is ensuring good positioning (perhaps always fixed at the top below a certain screen threshold) at small viewports.

Part of it is deciding which buttons are present. See also #705.

Part of it could be making the inline tools show up on select only, a la the mockups:

Probably necessary regardless — the ability to move some items into an overflow ellipsis menu.

[jasmussen]

Also this:

[jasmussen]

So, here's a mockup of how our various toolbars could behave in a responsive setting:

Not included is the freeform block toolbar, because that's still in flux and will need more time in the oven.

The quick toolbars have these states:

• Unscaled. Shown on wide viewports.
• Compressed. Space between control groups collapses.
• Responsive. Block level control groups collapse into a dropdown.

First question: can we achieve states 1 and 2 using flexbox?

Second question: can we achieve state 1 in a realistic amount of time?

In case question 2 is unfeasible, I have other ideas for how to tackle responsive toolbars, but this, I think is a good design. So very curious to hear your thoughts. CC: @youknowriad and @iseulde.

Can we use this polyfill to create CSS element queries? https://github.com/marcj/css-element-queries

[jasmussen]

This is rather improved in the most recent push. As such I'm switching milestones.

[jasmussen]

Master is working well currently. Closing this as fixed for now.