Polymorphize the OS specific details

[wtfbbqhax]

Cleanup the #if defined(JUCE_<OS>) from Main.cpp.

Maybe something like this to start

class PAL {
public:
    PAL() {}
    virtual bool elevate(const string args) = 0;
    virtual const char* platform() = 0;
    virtual const char* settingsPath() = 0;
    virtual const char* defaultBasePath() = 0;
};

class DarwinPAL : public PAL {
public:
    DarwinPAL()
    {
        settingsDirectory = appDataDirectory.getChildFile("Application Support").getChildFile("Tremulous");
        defaultBasepath = File::getSpecialLocation(File::globalApplicationsDirectory).getChildFile("Tremulous");
    }
    bool elevate(const string args);
    const char *settingsPath();
    const char *defaultBasePath();
}
bool DarwinPAL::elevate(const string args)
{ ... }
const char* DarwinPAL::platform()
{ return "osx"; }

...