Making use of Network mounted folders safer.

Hello, here you can find some changes to support network mounted folders

- first sync of each configured folder create a .inuse file in slotScheduleAllFolders() 
- the file is deleted on normal quit in Application Destructor folderman->unlockSyncedFolders

-if another instance of owncloud is launched on the same folder, it detects .inuse file presence and pause the folder sync process. Warning message message says it is dangerous.
- Resume button show the same warning message.
-Only a restart of the program will eventually unlock the folder if .inuse file is deleted by normal quit of syncing instance or manually.