lua : document email exporter

forgoten in the 1.4 doc, sorry about that
lua : remove default handler for types

this functionnality was unused and tricky to maintain
lua : implement proper inheritence in lua

this allows to properly handle c structs that extend other C struct
add -dev suffix to lua API version
Add a button to cycle through rating comparators

The list is <, <=, =, >=, >, !=
tonecurve revamped

we no longer clamp the a and b channel when using the corresponding curves.
instead an extrapolation is applied at both sides of the curve. this fixes
possible hue banding and artifacts mainly for images with certain highly saturated
colors (think of blue illuminated buildings at night, stage photos etc.)

as usual existing history stacks are kept untouched - if you want to take
advantage of the improvements for those you need to reset the tonecurve module.
colorcontrast revamped

same as before: no clamping of a and b channels.