Responsive cache generation

[wpferguson]

It is now possible to automatically, using events, selectively update the mipmap cache using the new API call dt_lua_image_t:generate_cache().

The script needs to register events for all actions that can change the image and regenerate the required cache images based on screen size and darktable settings.

[ChristianBirzer]

As I mentioned in #381 I'm currently trying to write a script that creates the mipmap cache of selected images.
I wouldn't start it automagically after image changes but trigger it with a button.
My idea is something to have like LR's 'build previews' command to allow the following workflow:

• import images
• calculate all previews/mipmap cache
• go for a coffee or two
• come back and work at full speed

Since I'm completely new to dt development, all I can do is to try my best 🤣. I'm not sure if it fits your needs and quality level. On the other hand, if you already begun working on it I don't have to do the same...