append .part to put files

This is an attempt to hande canceled file uploads that use PUT.

First, we store them, appended with '.part' (like chrome does while downloading files)
after we got the content we will rename the file to the original name

Second, to prevent synchronization duplicates of these .part files we ignore them when scanning files.

Third, @dragotin you should probably add "*.part" to the list of excluded files.

I could not reproduce the issue with GVFS. @dragotin @danimo maybe you could test this PR. @icewind1991 please review.