this is a complex fix that required to change some basic behaviours.
- the temp folder from blenderkit_data was moved to system temp folder
- There's a cleanup function for the old folder.
- search itself doesn't create files on drive(this rework will continue to try not to write also some other data, like gravatars, but cache them in mem only)

further fixes:
 - assetbar woudln't draw if there wasn't a thumbnail in an asset
 - categories panel poll function was fixed for brushes
 - fetching tokens from rerequests wasn't writing them into prefs

 TODO: create a popup that asks if categories can be downloaded and an example search can be performed.