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)

merge roster_contact_context_menu.glade, chat_control_popup_menu.glade and zeroconf_contact_context_menu.glade

 * 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

This commit does not improve overall structure in gajim in a logical way.
It seems untested (is broken) and is therefore rejected.

Resource conflict dialog has now a timeout. Fixes #4590

This kind of work will come after 0.13

This is the first commit of a long serious of cleanup commits.
Simple testcase included to test the slightly enhanced Interface dispatcher.

Looks like I screwed up the renaming.

The latter one has to execute the modules and does not work all the time.