[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: {F6800371}
Weather station page, after selecting a station: {F6800190}
Appearance page: {F6800372}
Units page: {F6800193}

The blank {nav 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