Clean up and standardize Settings dialog

Summary: Cleaned up and standardized Spectacle's Settings page, which was a bit bloated with unnecessary information and nonstandard formatting, terminology, and icons, and lacked any room to add any new settings (such as one for 372408)

Test Plan:
Tested in KDE Neon; all options still work, and Docbook is up to date with the new item labels. The best way to see what this patch does is to just look at before-and-after screenshots:

General page before:
{F5453807}

General page after:
{F5453820}

Save page before:
{F5453809}

Save page after:
{F5453821}

Reviewers: #spectacle, broulik, #kde_applications, #vdg, abetts, bgupta, jensreuterberg

Reviewed By: #vdg, abetts

Subscribers: bgupta

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D8530