sqlite with ? doesn't work as expected

prevent traceback when changing resource of a control when there is no event for the account at all. Fixes #5375

The CommonClass helps to spot easily where GC_Contacts and Contacts differ. This is just the first step a potential refactoring in this area.

Now we do:

contact.supports(feauture) instead of gajim.capscache.is_supported(contact, feature)

 * Rename EntityCapabilities to ClientCaps as this seems more intense giving.
 * Add ability to blacklist features where we cannot savely assume that a client, which did not advertise caps, supports them

Usage:

	if contact.supports(NS_E2E):
		....

Implement and test a supports_feature() method to directly test for supported featues on EntityCapabilites