Add Desktop and Downloads to the default list of Places

Summary:
Add Desktop and Downloads to the default list of places, for the following reasons:

1. The Desktop and Downloads folders are heavily used by average users, and making them accessible by default from Dolphin and file open/save dialogs is a big usability win
2. We'll have space to do it without cluttering up the panel from having removed some un-useful entries in D11767: Remove Recently Saved This Month and Recently Saved Last Month entries by default

FEATURE: 203472
FIXED-IN: 5.46

Test Plan:
Created a new user account, logged into it, and opened Dolphin:

Reviewers: Dolphin, Gwenview, Frameworks, VDG, markg, progwolff

Reviewed By: markg, progwolff

Subscribers: abetts, huoni, markg, fabiank, progwolff, broulik

Tags: Frameworks

Maniphest Tasks: T8349

Differential Revision: https://phabricator.kde.org/D11768