lua : document email exporter

forgoten in the 1.4 doc, sorry about that
(cherry picked from commit 7d81a8f7301b39f37c53b5f98b3260704ea45359)
lua : remove default handler for types

this functionnality was unused and tricky to maintain
(cherry picked from commit 6eb02c3ffa899ca1f632945a0f00c0ba356530dc)
Add a button to cycle through rating comparators

The list is <, <=, =, >=, >, !=
(cherry picked from commit 9d73443063e4f02580ffd9ab759448680c6a0266)
shadows&highlights revamped

main improvement is elimination of clamping where possible - no clamping of the
module's input and no clamping of its output. this should help to solve two issues:

* prevents hue banding when working on images with very high color saturation
* highlight reduction now also works on out-of-bound highlights

minor changes: lower memory usage in CPU codepath; better numerical stability on GPU.

legacy history stacks work as before - in order to switch to the new algorithm for
them you need to reset the module.
(cherry picked from commit 9ad7489e35b77ac96ad80f25442019250e49848c)