ZBX-1096 changed the behaviour so now selected entity list is always reset after performing an action. but actually remembering turned out to be useful in some cases...
for example, if one would like to change refresh period and enable a bunch of items, previous behaviour allowed that without recreating the selection.

this remembering should be "smart" - it should only show entities that are still visible and existing. possible reasons for entities not to be visible anymore - they have been deleted or changed so that filter or subfilter excludes them.

additionally, last selected page should be remembered as well.