Rescan folder using MediaScanner after state flips from syncing to 100% idle

[Catfriend1]

Description of the issue

syncthing/syncthing-android#1488

We need some pretty small and effective code with good error catching to do the following:

• If a folder state transitions from "syncing" to "idle 100%" check if .nomedia files are present in the folder
• If one or more of them are present, rescan the folder with MediaScanner.

[bell07]

My proposal is to remember during sync if any ".nomedia" file was added, changed or deleted. If they was such change, trigger the full rescan of the folder that contains the changed file, after the sync was completed.

Example: Syncthing folder is MySync. File "MySync/Photos/random/.nomedia" was created => result is rescan of "MySync/Photos/random/" after MySync was fully synchronized.

[Catfriend1]

Yes, the idea is good, but I currently can't make up my mind for a light implementation.