Modernize Settings window

Summary: This patch modernizes the appearance of Dolphin's configuration window by following the KDE HIG as much as possible and following design cues from Plasma and System Settings.

Test Plan:
Tested all settings to make sure they still work; they do.

Startup page, before:

Startup page, after:

View page (Icons), before:

View page (Icons) after:

View page (Compact), before:

View page (Compact) after:

View page (Detailed), before:

View page: (Detailed), after:

Navigation page: no change

Trash page, before:

Trash page, after:

(Provided by D12986: [Trash KCM] Clean up and standardize UI to be in line with the KDE HIG)

General page (behavior), before:

General page (behavior) after:

General page (confirmations), before:

General page (confirmations), after:

General page (status bar): no change

Reviewers: Dolphin, VDG, broulik, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: fabianr, cfeck, medhefgo, zzag, rkflx, kfm-devel, elvisangelaccio, abetts

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D12571