ComboGrid: open picker also when focusing an editable field

ComboGrid: open picker also when focusing an editable field

port commit
851c032d69ad5ae23725dd1add9e4084ebc12650
from pve-manager

commit message:

    ComboBoxes provided by UI frameworks normally show their selection
    possibilities once focused, ExtJS does this too but not on editable
    (and thus filterable) input fields. Add a logic which allows this too
    and streamlines the behavior of ComboGrid/Box like components.

    To avoid a glitch, where clicking the "picker trigger" (the down
    arrow which normally toggles the selection picker) would first focus
    the element - thus expanding it - and only then causing a toggle
    which collapses it again, we overwrite the 'Ext.form.trigger.Trigger'
    onMouseDown so that we can skip a expansion on this event.

    Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>