InputComponent: add a setting to disable SDL

In some situations it appears that the SDL code causes issues (such as
crashes), even if the user isn't using SDL. This new setting is supposed
to help debugging this. A user can set main.sdlEnabled to "false", and
we won't initialize and use SDL.

src/input/InputComponent.cpp

@@ -91,7 +91,8 @@ bool InputComponent::componentInitialize()
   addInput(new InputAppleMediaKeys(this));
 #endif
 #ifdef HAVE_SDL
-  addInput(new InputSDL(this));
+  if (SettingsComponent::Get().value(SETTINGS_SECTION_MAIN, "sdlEnabled").toBool())
+    addInput(new InputSDL(this));
 #endif
 #ifdef HAVE_LIRC
   addInput(new InputLIRC(this));