Add string.commands
Add stubs to string.c
Add string files to CMakeLists.txt
Add ws_builtin_cmd_strcat implementation
Add ws_builtin_cmd_substr implementation
Add static function get_strings_from_union
Add implemenation of ws_builtin_cmd_strcmp
Add error output to main.c
Merge pull request #364 from xVin/Add-error_output-in-main

Add error output to main.c
Merge pull request #362 from xVin/Add-string_commands

[WIP] Add string commands
Add extensive documentation about the hotkey subsystem
Add initial hotkey event type
Add (de)initializing functions
Add file for hotkey DAG implementation
Add interface definitions and stubs for DAG
Add function for retrieving the next DAG node
Add implementation of insertion function for the DAG
Add preliminary implementation of deinitialization
Add implementation of event removing function
Add initial setup for hotkey subsystem