[4.x] Assorted fixes

[jasonvarga]

• Consistent spacing in pane-style forms. (Nav item editor, CP section and item editor)
• Revert overzealous section/tab find/replace within nav components that happened in [4.x] Blueprints: Rename sections to tabs #7533. Those "sections" are still "sections".
• The Popover component provides an object containing itself, so children (in a portal) can reference it. Then use use that to close the popover from within dropdown list. Now that the DropdownItem is within a portal, $parent was not what it used to be, which caused an error when clicking an item.
• Fix error when clicking the topmost "Update" button in the updater.