Reduce string conversions

[badcel]

The UTF8-Strings and Filename / Platform strings are currently always converted to C# strings. A user of the library perhaps does not want to display the path to the user but just use it as an input for another method.

In this case a method should probably accept a new type like "UTF8String" or "PlatformString" and there could be an overload which takes a C# string and does the conversion and calls the UTF8String based method. So the user can decide which method to use.

Idea came up as part of: https://github.com/PintaProject/Pinta/issues/1616#issuecomment-3070684867