Test: fix some warnings and errors

See merge request !131

_registered_name is None on a smacks resume.

Gajim creates on every connection attempt a new nbxmpp.NonBlockingClient
instance. This is why _registered_name is None when we go through a
smacks resume, because there is no bind event, and the new NonBlockingClient
instance has no knowledge of the previous successful bind.

we need this for automake as it seems

- Move Credits to const.py
- Minor Refactoring

Add issue templates

See merge request !127

gtkgui_helpers: remove some unused code

See merge request !125

common.helpers: simplify get_os_info()

See merge request !122

add pylint run to CI

See merge request !123

Replace @nested_property with @property in dataforms

See merge request !124

fix some import warnings and errors pointed out by pylint

See merge request !121