[System Monitor] Port settings window to QQC2+Kirigami FormLayout and modernize UI

Summary:

• Port to QQC2+Kirigami FormLayout style, except for the SpinBox which cannot handle decimal values well in the QQC2 version
• Use standard import names
• Adjust some strings to sound a bit more natural

Test Plan:
Tested with all the variants (CPU, network, etc). All features still work and settings
are saved and loaded properly.

Reviewers: VDG, Plasma, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D21074