Modernize Settings window

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:
{F5825313}

Startup page, after:
{F5918574}

View page (Icons), before:
{F5825319}

View page (Icons) after:
{F5918575}

View page (Compact), before:
{F5825321}

View page (Compact) after:
{F5918700}

View page (Detailed), before:
{F5825323}

View page: (Detailed), after:
{F5918701}

Navigation page: no change

Trash page, before:
{F5858748}

Trash page, after:
{F5866656}
(Provided by {D12986})

General page (behavior), before:
{F5825316}

General page (behavior) after:
{F5918572}

General page (confirmations), before:
{F5866885}

General page (confirmations), after:
{F5918702}

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