[sql lab] improve TableElement controls

[mistercrunch]

Making TableElement (the table metadata shown in left panel in SQL Lab) show
controls only on hover, and use caret instead of plus/minus icons to expand/collapse

CATEGORY

Choose one

• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

[codecov-io]

Codecov Report

Merging #7881 into master will decrease coverage by 8.01%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master    #7881      +/-   ##
==========================================
- Coverage   73.78%   65.76%   -8.02%     
==========================================
  Files         107      461     +354     
  Lines       11613    22176   +10563     
  Branches        0     2425    +2425     
==========================================
+ Hits         8569    14585    +6016     
- Misses       3044     7470    +4426     
- Partials        0      121     +121

Impacted Files	Coverage Δ
...rset/assets/src/SqlLab/components/TableElement.jsx	84.33% <54.54%> (ø)
superset/assets/src/components/Checkbox.jsx	100% <0%> (ø)
...ations/deckgl/layers/Polygon/PolygonChartPlugin.js	0% <0%> (ø)
...ets/src/dashboard/components/dnd/DragDroppable.jsx	94.59% <0%> (ø)
...c/visualizations/deckgl/layers/Polygon/Polygon.jsx	0% <0%> (ø)
superset/assets/src/components/EditableTitle.jsx	81.53% <0%> (ø)
superset/assets/src/setup/setupPlugins.js	0% <0%> (ø)
...t/assets/src/components/InfoTooltipWithTrigger.jsx	41.66% <0%> (ø)
.../src/dashboard/components/UndoRedoKeylisteners.jsx	9.52% <0%> (ø)
...ts/src/explore/components/ExploreActionButtons.jsx	100% <0%> (ø)
... and 345 more

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[graceguo-supercat]

LGTM