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

Summary:

• Port to QQC2+Kirigami FormLayout style
• Adjust some strings to sound a bit more natural
• Hide "Compact Mode" header since all the settings currently apply to compact mode; it only makes sense to display this when there's an additional mode to contrast with it

Test Plan:
Weather station page, before selecting a station:

Weather station page, after selecting a station:
Appearance page:
Units page:

The blank Visibility combobox is a pre-existing bug not introduced (or fixed)
with this patch.

Reviewers: VDG, Plasma, kossebau, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20896