Fix text synchronization in locales that use "," as decimal separator

Thanks to @hansnolte for pointing out this bug
Text up/down control: fix bug when toggling enable state
Tone-mapping dialog: overhaul interface

Relates to #149.

Each tone-mapping technique now has its own panel, and user-customizable
options are exposed for each.

Also, linear conversion now defaults to a gamma value of 2.2.

Many thanks to @hansnolte for his invaluable help on this topic.
HDR export: apply 1/2.2 gamma prior to saving

Relates to #149.

This seems to be the default behavior for other HDR files, and I would
like PD to behave similarly.  With this change, images exported from PD
can now be losslessly imported at the default 2.2 gamma value, rather
than requiring special 1.0 gamma treatment.
README: update to 6.6a and add (awesome) new contributors

Many thanks to all the great people who have contributed to this
release!