Add an anonymous mode to flow launcher

### Discussed in https://github.com/Flow-Launcher/Flow.Launcher/discussions/1974

See also the [original issue](https://github.com/Flow-Launcher/Flow.Launcher/issues/1947) by @skotarek

Reopened this as an issue given the opinions in the discussion that this is implementable.

This could perhaps be separated into different feature requests but would be more of a general aim at the moment.
Not sure if there's any real way to guarantee plugins don't store a history (other than some way to trick them into writing to memory not disk or something)
But at least in terms of the launcher + core plugins this could probably be done, and we could consider things such as wiping parts of the user data folder on exit, exposing a setting that plugins should respect etc

Start with Discussion #1974 and the original issue #1947 to understand the proposed privacy scope. Identify how the launcher and core plugins store cache and history, then define which data must be cleared or disabled and how plugin settings should be respected. Done should include an agreed, testable anonymous mode scope rather than an unspecified collection of privacy features.

Written by the indexing model from the issue text.