ui: ComboBoxSetStoreNode: don't hide the picker when clicking the toolbar

ui: ComboBoxSetStoreNode: don't hide the picker when clicking the toolbar

When clicking the toolbar of the ComboGrid, the combobox loses focus,
and instantly hides the picker.

To prevent that, we keep track of the mousedown event on the toolbar
(which happily comes before the focusLeave event), and prevent the
focusLeave propagation in that case.

Then on mouseup, we focus the combobox again, so that the nexct
focusLeave can trigger again.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>