Add separate icon keys for .NET solution and project files.

Release Notes:

- Icon themes: Added the ability to change file icons for Visual Studio
project files:
  - Solution files (`.sln`)
  - Solution User Options files (`.suo`)
  - C# Project files (`.csproj`)
  - F# Project files (`.fsproj`)
  - Visual Basic Project files (`.vbproj`)

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Co-authored-by: Marshall Bowers <git@maxdeviant.com>