refactor database query helpers
make download file function foreceable
handle file overwrite case when a file is modified
Merge branch 'master' of github.com:cozy-labs/cozy-desktop
change applyFolderDBChange

It creates missing dirs in the database instead of deleting them.
make applyFolderDBchanges function less wide
Merge branch 'master' of github.com:cozy-labs/cozy-desktop