Item search filters - Multi type/collection

[luku]

Hi, this could be possible way to display search filter for multiple item types and/or multiple collections. However, for collections it requires PR #626 to be accepted first. Also not sure if I should rather update my PR #707 directly instead (there every item type/collection will need to be separate filter, not like here comma separated). But in the case it will not be accepted anytime soon, this could work. I also added multiple select for item types and collections in advanced item search