Created attachment 311255 [details] [review]
Prototyping patch.

This is a feature request. Since I'd like to switch quickly through out a few tool options quickly without any click, while currently tool presets do not allowed keyboard accesses. 
I also found a similar request here. https://mail.gnome.org/archives/gimp-user-list/2012-November/msg00026.html
Actually, I have already made a rough patch based on latest codebase. Just wondering would Gimp ever consider to add this feature in?
The behaviours I'm using are mainly inspire by MyPaint.
It look like this:
1. New actions tool-presetN-save/ tool-presetN-load actions are available to users in keyboard configurations. 
2. Users should stroke the binding key for tool-presetN-save first. A new preset is made up based on current tool options named "CachedPresetN".
3. Whenever user stroke tool-presetN-load binding key, select the tool having CachedPresetN, and switch to that preset.
4. Cached presets are keep across application sessions.

* Note:
1. I didn't add binding key by default in this patch, since some number keys are already assigned to zoom and recent file for Ctrl modifiers Maybe users want this feature need to configure them for themselves.
2. The caching presets are controllable like other regular presets, I though I supposed to make them grey out, but I don't know the codebase enough to do this.

BR
CY