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

Fix findbugs method doesn't start with a lower case letter error in
frontend EntityModel class methods OnEntityChanged, EntityChanging,
entityPropertyChanged, ValidateEntity and AsConvertible

Change-Id: I67fcaae18631cde49ede25d1a4faee6a4209bdc0
Signed-off-by: Tal Nisan <tnisan@redhat.com>