Round 2 of Preferences layout improvements

This MR applies the suggestions of @alvinwong in !1328 (comment 397209) about grouping some of the sets of checkboxes.

I've also changed some repetitions of strings in "Swap File" panel, and added a scrolling area for the Miscellaneous tab of the General pane; I'm running on height limitations on my 13" Macbook Pro.

Relevant portfolios:

• Before !1328 (merged): https://imgur.com/a/Klc3eaN

• After !1328 (merged): https://imgur.com/a/EHi07LZ

• After this MR: https://imgur.com/a/WkOcp76

Test Plan

Build Krita.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

/cc @alvinwong