remove assumption in plugin.applyChanges() that a file exists

[samuelstroschein]

Context

applyChanges() assumes that a file exists. That is not always true.

• a lix syncing might receive changes from one client that constructs a file
• a file might have been deleted and should be reconstructed

Proposal

Remove the assumption that a file exists when calling plugin.applyChanges(). Plugins would need to create a new file if no file exists. Seems seems OK.

Additional information

files are computed data from changes. at no point other than opening a file with an app or getting a file out of lix is applyChanges needed. Hence, letting plugins create file (if they don't exist) seems to make sense.