Don't update shadow values on mini toolbox blocks when mini toolbox is closed

[ajpal]

There was a rendering issue in some browsers (Firefox, IE, Edge):

The root cause is that getComputedTextLength() returns 0 if the text element is not visible. So basically, when we updated the object block, the "object" label thought its width was 0, so the preview went on top of it. The solution is to not update the minitoolbox blocks when the flyout is closed, and this corresponding PR updates them when the flyout opens

This PR also renames tray to isMiniFlyoutOpen

[ajpal]

there's no change to the logic here, I just think this is a little more readable.
this.block_.miniFlyout will be truthy for any block that has a minitoolbox, regardless of whether it's open. this.block_.tray (renamed to this.block_.isMiniFlyoutOpen) is true if the flyout is open.
so before it was

if (flyout is open) {
  show()
} else if (there is a flyout) {
  hide()
}

And now it is

if (there is a flyout) {
  if (it's open) {
    show()
  } else {
    hide()
  }
}

[codecov-commenter]

Codecov Report

Merging #228 into master will decrease coverage by 0.01%.
The diff coverage is 5.00%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
- Coverage   59.46%   59.45%   -0.02%     
==========================================
  Files          95       95              
  Lines       11697    11699       +2     
==========================================
  Hits         6956     6956              
- Misses       4741     4743       +2     

Impacted Files	Coverage Δ
core/ui/block.js	71.07% <0.00%> (-0.13%)	⬇️
core/ui/block_svg/block_svg.js	83.50% <16.66%> (-0.15%)	⬇️
core/ui/block_space/block_space.js	65.82% <0.00%> (+0.21%)	⬆️

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