Changing from char* strings to std::string strings

If you look back far enough, 'plcd' was written in C and was refactored over time into C++.

One upshot of this being that there are many instances of doing things the old C way rather than using an STL construct. No where is that more apparent than in the use of char* strings.

I'm going to fix this by using std::string for strings wherever possible.

As a benefit to this, Commands will have generic SetParameter and GetParameter functions that use a std::map to store the data in. This will enable Command::Clone() to copy all the settings from one command to another.

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