Support for panel-controlled backlights

[tylercamp]

(Moved from #583 )

Some panels, i.e. SSD1963, come with built-in backlight control which require device-specific commands to manage.

Currently there isn't any ILight implementation which supports this.

---

For SSD1963 specifically, these commands are needed for backlight control:

  void Set_1963_PWM(uint8_t value)
  {
    startWrite();
    writeCommand16(0xBE);
    _bus_instance.flush();

    writeData( 0x05 );
    writeData( value );
    writeData( 0x01 );
    writeData( 0xFF );
    writeData( 0x00 );
    writeData( 0x00 );

    endWrite();
  }

---

Since this type of backlight control is specific to the particular Panel_LCD, I wonder if any of these minor refactors would be appropriate:

// additional class to be mixed in when appropriate and manually called by the user if needed
class IPanel_LCD_InteractiveBacklight {
   // (1) panel is capable of creating an ILight instance which references this panel for commands
   virtual ILight* createLight() = 0;
   
   // ... or ...
   
   // (2) panel directly exposes a `setBrightness`, and a new `Light_InteractivePanel` class
   //       would simply defer to this
   virtual void setBrightness(...) = 0;
};

// ... or ...

class Panel_SSD1963 {
public:
   // (3) panel defines its own `ILight` impl to be explicitly created by the user and assigned to `LGFX_Device`
   class Light : ILight { ... };
};

// ... or ...
class Panel_Device {

   // ...
   
   // (4) all panels may directly offer a `ILight`; panels like SSD1963 will always have backlight control
   //
   // if no `ILight` was specifically assigned to `LGFX_Device`, then it would defer to 
   // the panel
   virtual ILight* getLight() { return nullptr; };
   
   // ...
};

(2) seems most in line with current LGFX patterns