ui: fix not opening 'bulk action' windows

ui: fix not opening 'bulk action' windows

we previously removed the 'filters' property from the store, but this
has the effect that the filter array is only ever initialized when
we try to access them via the specified api of extjs

so instead of adding them manually to their array, use the 'addFilter'
method of extjs

also use 'getStore()' instead of directly accessing the store to be
consistent with the extjs api use

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