GH-1060 update tweaks

* download to multimc folder hierarchy
* use rename, not copy
* keep backup after update
* clean previous backup before update
* it's not 'copy', it's 'replace'

logic/updater/DownloadTask.cpp

@@ -27,8 +27,8 @@
 namespace GoUpdate
 {
 
-DownloadTask::DownloadTask(Status status, QObject *parent)
-	: Task(parent)
+DownloadTask::DownloadTask(Status status, QString target, QObject *parent)
+	: Task(parent), m_updateFilesDir(target)
 {
 	m_status = status;