Move containers utils to containers.hh and add filtered/transformed utils
Unify completion from container content logic
Remove prefix_complete, superseeded by complete
Extract complete_buffer_name as a free function in commands.cc
Code refactoring in WordDB
Tweak id_map implementation
Use an id_map directly for HighlighterRegistry rather than the FunctionRegistry class
Rename id_map to IdMap
Remove trailing blank lines
sort includes
Minor code tweak in optional.hh
Style tweak
Refactor color functions
Add a join function for joining strings using a specific char