Allow transitioning to colour temp for tradfri

[Hate-Usernames]

Description:

Adds transition time when changing colour of a tradfri light.

Related issue (if applicable): fixes #

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.github.io#<home-assistant.github.io PR number goes here>

Example entry for configuration.yaml (if applicable):

tradfri:

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[lwis]

Looks good, must have missed this when I did my refactoring. 🐊

[poldy79]

I just upgraded to version 0.69. Transition is working for brightness but not for kelvin. Color changes instantly, brightness as expected. Any suggestions?