Context Menus - add keyboard navigation within a menu

[eug3nix]

Currently there are two issues with context menus:

1. When context menu is shown with a mouse right-click it it does not have a keyboard focus.
2. The user should be able to navigate the active context menu using keyboard errors and the enter key to select/activate menu item
3. The displayed context menu should close when escape is pressed

[plucik]

Context menu support implemented in tabulator_context_menu_mixin.js and works in Data Editor grid and QueryResultsTab grid.
In the database tree keyboard navigation within the context menu works but UI is broken because we use custom itemRender in showContextMenu method of power_tree.js mixin.
We can use showContextMenu without custom itemRender, but doing so means losing the feature we implemented - wait for a delay to prevent the menu from being hidden due to the user moving the mouse too fast.