Components: Extract BlockToolbar as a reusable component

[youknowriad]

Related to #2761 (comment)

This PR extracts BlockToolbar component to the reusable components folder by removing all layout styles from the reusable components.

Notes

• This also extracts BlockMover, BlockSettingsMenu and BlockSwitcher. These are components used by the BlockToolbar and also by another reusable component (BlockList not yet made reusable), so I decided to move them to the reusable components folder but without exposing them in wp.editor because reusing those explicitely to build alternative editor layouts makes no sense, we reuse BlockToolbar or BlockList instead of including those directly.

Testing instructions

• Test that the Block Inspector works and looks like master.

[codecov]

Codecov Report

Merging #3491 into master will increase coverage by 0.18%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3491      +/-   ##
==========================================
+ Coverage   34.64%   34.82%   +0.18%     
==========================================
  Files         260      260              
  Lines        6737     6704      -33     
  Branches     1219     1215       -4     
==========================================
+ Hits         2334     2335       +1     
+ Misses       3717     3687      -30     
+ Partials      686      682       -4

Impacted Files	Coverage Δ
...ponents/block-settings-menu/block-delete-button.js	0% <ø> (ø)
...omponents/block-settings-menu/block-mode-toggle.js	44.44% <ø> (ø)
...omponents/block-settings-menu/unknown-converter.js	0% <ø> (ø)
editor/components/block-mover/index.js	10% <ø> (ø)
editor/components/block-toolbar/index.js	0% <ø> (ø)
...ents/block-settings-menu/block-inspector-button.js	0% <ø> (ø)
editor/components/block-mover/mover-label.js	100% <ø> (ø)
editor/components/block-switcher/index.js	2.85% <ø> (ø)
editor/components/block-settings-menu/index.js	0% <0%> (ø)
...or/modes/visual-editor/block-contextual-toolbar.js	0% <0%> (ø)	⬆️
... and 9 more

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