COMMON: Introduce --debug-channels-only command line flag.

Many of our systems currently generate significant amount of debug
output on deeper levels. Now, when your engine is using Debug Channels,
you might want to show that debug information only, which is currently
not possible, as the generic output will be mixed in your output.

Alternative solution would be to implement possibility to specify
per-channel debug levels.

base/main.cpp

@@ -391,6 +391,10 @@ extern "C" int scummvm_main(int argc, const char * const argv[]) {
 	} else if (ConfMan.hasKey("debugflags"))
 		specialDebug = ConfMan.get("debugflags");
 
+	if (settings.contains("debug-channels-only"))
+		gDebugChannelsOnly = true;
+
+
 	PluginManager::instance().init();
  	PluginManager::instance().loadAllPlugins(); // load plugins for cached plugin manager