Tag suggestions

It would ease user's life if the app would suggest tags and user could just apply them.
This would come in 2 forms:
1. "Quick tags" section in Gallery's tags edition dialog.
Right now, "quick tags" don't contain resource-specific tags, only collection-specific (the most popular tags).
We can also add "suggestions" into "quick tags".
2. Resource Grid screen can have pop-up dialog, displayed right after indexing is over.
The pop-up would list unlabeled resources with "suggestions" next to them.
User can approve or ignore them on per-tag basis.

"Suggestions" can be generated by similarities found.
Image-based similarity search would consume too much power for mobile device, so this criteria is left for future Desktop/Server implementations. Lightweight search would include the following parameters:
* parts of filename, Levenstein distances
* EXIF-metadata of images
* text content of documents

_Requires better specification_

No files, tests, or entry points are named. First clarify whether the work covers Gallery quick tags, the post-indexing Resource Grid dialog, or both, and define the supported suggestion inputs and approval behavior. Done should be a documented, agreed specification before implementation begins.

Written by the indexing model from the issue text.