[kcms/nightcolor] Minor UI tweaks to the KCM

Summary:
- the slider is now of the same width as the comobox below it
- the slider's value has been placed to the right of the slider, as the HIG mandates
- use title case for "Detect Location" button
- add an icon to the "Detect Location" button

Test Plan:
Before
{F6886139}

After
{F7128957}

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: zzag, romangg, GB_2, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22863