lots of keyboard accessibility fixes for gridpicker

[riknoll]

fixes https://github.com/microsoft/pxt-minecraft/issues/2806

adds a bunch of fixes related to using the gridpicker with keyboard controls in minecraft:

• pressing escape when the field editor is expanded no longer opens the simulator
• fixes the focus indicator outline on shadow blocks with dropdown fields and labels, but no other editable inputs/fields (like the item block in the linked issue)
• focus is now trapped in the gridpicker when either the searchbar or selection bar is present
• the selected item is now actually visible when selecting using the keyboard arrows (uses the standard focus color)
• tabbing out of the searchbar no longer resets the selected item
• fixes keyboard nav when the items in the grid are filtered via the searchbar
• the tooltip is now displayed underneath the selected item when using keyboard navigation