Port common.branch segment to KwThreaded*, add common.repository_status
Allow multiple shutdown calls
Allow multiple starts by not subclassing from Thread
Fix behavior with run_once: .update() was called too much times
Make update_first configurable
Rework API for custom vim events
Use python’s try/catch, not vim’s

Fixes #344
Move configuration to /etc/xdg/powerline

XDG_CONFIG_DIRS is set in /etc/env.d by configuration from xdg-utils package, 
but it is not guaranteed to be present in the system
Merge @ZyX-I threaded-3 branch, resolving conflicts