Add new collection filter to find exact matches

Searching for label:=pop finds songs labeled with 'pop' but not with
'electro pop'. While searching for label:pop finds songs labeled with
'pop' and 'electro pop'.

GUI is also updated.

REVIEW: 102252

GUI: edit collection search and edit dynamic playlist dialogs changed
slightly
DIGEST: Feature: new "exact" field:=value match in collection search
FIXED-IN: 2.5