ComboGrid: open picker also when focusing an editable field

ComboGrid: open picker also when focusing an editable field

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>