Add support for better environment variables in the framework, because since today it was not possible to start a process and control which environment should have:

• Added functions to SystemStats to allow getting and changing the environment:

bool setEnvironmentVariable(const String& name, const String& value)

bool removeEnvironmentVariable(const String& name)

StringPairArray getEnvironmentVariables()

• Added functions to ChildProcess to pass the environment variables the child process should retain:

bool start (const String& command, const StringPairArray& environment, int streamFlags = wantStdOut | wantStdErr)
bool start (const StringArray& arguments, const StringPairArray& environment, int streamFlags = wantStdOut | wantStdErr)

• Added a function to Process to pass the environment variables the process spawned should retain:

static bool JUCE_CALLTYPE openDocument (
    const String& documentURL,
    const String& parameters,
    const StringPairArray& environment)