Combobox keyboard support
    
    Combobox can be controlled just by using keyboard.
    
    When value list is closed, user can:
      * use UP and DOWN error to open list, it will focus the list and
        select previous/next value
      * when CB is non-editable, user can start typing, first character will open
        list, second will be entered into search input. Note: I wanted to copy the
        first char to the search box as well, but I did not figure out reliable
        method for converting keycode to char for non ASCII keyboard layouts
      * ESCAPE, ENTER, TAB keys are handled to allow keyboard operations in a
        container
    
    When value list is opened:
      * CB tries to keep focus on either search input or a select
      * when focus is lost, the value list is closed. So  user can click anywhere
        on a page to close it - two comboboxes can't be opened on the same time
      * hitting TAB key switches between search and select
      * if CB is not searchable, hitting TAB will close the value list and select
        input textbox
      * hitting ESCAPE on will close the value list
      * hitting ENTER on search input will invoke search operation
      * hitting ENTER on select will close the value list
      * hitting UP/DOWN arrows will select previous/next values
    
    Additional modifications:
      * opening arrow and search button were made non-focusable. It fixes the
        'wrong focus area' bug and simplifies keyboard usage. It doesn't affect
        mouse usage.
    
    https://fedorahosted.org/freeipa/ticket/3324