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Move ensure server to the very beginning

Also, fix documentation for a slot and mini-refactor a method
for improved translations.
detection of multiple instances using flock.

sending RAISE_WINDOW leap event if we are not
the main instance.
remove duplicated ensure_server
Use dark icons (light theme) for OSX

Closes:#2130
missing comma in tuple
Merge remote-tracking branch 'kali/feature/single-instance-app2' into develop

Conflicts:
	src/leap/gui/mainwindow.py
Emit session_id and uid through events