[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

After

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: zzag, romangg, GB_2, plasma-devel

Tags: Plasma

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