frontend: Fix findbugs method doesn't start with lower case

Fix findbugs method doesn't start with a lower case letter error in
frontend ListModel class methods ItemsChanging, ItemsChanged,
UnsubscribeList, SubscribeList, ItemsCollectionChanged,
ValidateSelectedItem and SelectedItemChangedEventDefinition property

Change-Id: Iab6b62a4e9806805b3282ed653ac1e4aa74768e4
Signed-off-by: Tal Nisan <tnisan@redhat.com>