Added class diagram from SchoolMessOrganizer repo
Removed usage of smo static functions from libsmo
Made smo a normal (non-singleton) class
Added (incomplete) database diagram
Removed smo class, adjusted init_grade_system_vector
to not cause errors when called multiple times
Removed xml classes
Removed last references to the smo class
added learn_period_container, removed some static attributes
Fixed learn_period::name_valid,
made learn_period_container::get_map const
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Added MySQL Workbench database model, removed old dia model, renamed model folder
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Completely removed sigc++, most API changes for the new SQL stuff done
Removed salt from database as it is stored in the bcrypt hash, added bcrypt hash generation
Merge commit 'b21e8b2af3594286d6a8ece7bdd11bda6cedfbfb'

Conflicts:
	libsmo/include/user_course.hpp
Added a bit new API that I found will be needed, some work on making smo-gtk use the SQL backend
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More work on making the frontend work with the SQL backend, removed grades table