Moved list of mods logic to a new CModManager class.
The list of enabled mods is now managed by a flag instead of directory
names of mods.
Mods are now disabled by default.
Also general cleanup, fixing issues from the code review in
https://github.com/colobot/colobot/pull/1191 and fixing linter issues.
Regression: the state of enabled/disabled mods is now not persistent.
The plan is to use some kind of config file for this.
This avoids specializing CSingleton<T>::m_instance for each type, and
instead just defines it once in the header. This is allowed by the
standard, multiple definitions are merged in the same way that inline
functions are.
MrSimbax