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}

FEATURE: 203472
FIXED-IN: 5.46

Test Plan:
Created a new user account, logged into it, and opened Dolphin:
{F5771426}

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