Add a user-facing control to choose the ICC color rendering intent

Summary:
FEATURE: 359909
FIXED-IN: 18.04

Allow the user to choose the ICC color rendering intent, instead of hardcoding INTENT_PERCEPTUAL

Supersedes D8763

Test Plan:
The GUI control appears and seems to work as intended.

{F5680320}

- Tested that the default value of Perceptual is used when there is no value in ~/.config/gwenviewrc
- Tested that the value gets set in ~/.config/gwenviewrc
- Tested that removing the value in ~/.config/gwenviewrc reverts the GUI setting to Perceptual
- Tested that toggling the setting back and forth actually has an impact when using a display with an active color profile. Here's an example:

Perceptual:
{F5677933}

Relative Colorimetric:
{F5677990}

Reviewers: DrSlony, rkflx

Reviewed By: rkflx

Subscribers: muhlenpfordt, rempt, behrmann

Differential Revision: https://phabricator.kde.org/D10076