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 OnSelectedItemChanging,
OnSelectedItemChanged, SelectedItemsChanged, SelectedItemsChanging,
SelectedItemPropertyChanged, ItemPropertyChanged

Change-Id: If62a800faa093253a336ef181f6c5af4d873769b
Signed-off-by: Tal Nisan <tnisan@redhat.com>