Draft: Introduce heuristics on when to ask the user to index a folder

Some folders are obviously (to humans) stuff that shouldn't be indexed. Manually blocking every folder that shouldn't be indexed from indexing can be a chore, and missing a folder can lead to situations to where Baloo is expending considerable resources on files the user wouldn't want indexed.

This introduces heuristics on common folder types users don't want indexed, e.g. source code or program installation folders, and a mechanism that will cause Baloo to focus its efforts on folders that it's more confident the user would like to see indexed, placing folders it's unsure about on the backburner. When it finishes indexing, it'll present the user a list of folders it was unsure about, and ask them if they would like them indexed or not.

Draft because actually asking the user isn't implemented yet. Not sure where to put the GUI.

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