Add "disable addins" buffer flag and fix a critical in the grep plugin

Merged Vanadiae requested to merge Vanadiae/gnome-builder:fix-slow-replace-grep-plugin into master

This allows disabling features that really are just useful when the buffer is displayed in a source view. That way we can avoid having e.g. codespell burning the CPU/disk when doing a Search and Replace on a project…

Fixes #1492 (closed)

Note though that while it means codespell won't burn the CPU/disk anymore, all buffer (manager) functions only work single threaded and hence I can't use a worker thread. So it's still slow and unresponsive on my computer, but no idea how to improve that further since a worker thread won't work.

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