Improve auto completions

[deathaxe]

This PR introduces the following changes to improve auto completions:

1. Load .tags file relative to active view, the same way as for GotoSymbol
2. Directly iterate through .tags file without using awk.
3. Restrict auto completions to .tag files with 100MB of size or less.
4. Drop a lot of type conversions between lists and sets.
5. Don't return completions for metadata !_TAG_... lines.
6. Cache completions per tag_file. Doing so globally may have unwanted side effects when multiple projects/windows are open.
7. Drop extracting and merging with ST completions, as merging all sources is already performed by ST. No need to do it here.