Items of Places menu should be configurable

[Norbert]

#### Expected behaviour
Some users [expect](https://ubuntu-mate.community/t/how-to-customize-places-menu/25116) that at least the "Network" and the "Connect to Server..." items are configurable via some relevant gsettings/dconf keys.

#### Actual behaviour
The mentioned and other items of Places menu are currently hardcoded, the do not have any relevant gsettings/dconf keys.

#### Steps to reproduce the behaviour
1. Install Ubuntu MATE
2. Try to configure Places menu

#### MATE general version
1.26.0

#### Package version
1.26.2-1

#### Linux Distribution
Ubuntu MATE 22.04 LTS

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: mate-panel 1.26.2-1
ProcVersionSignature: Ubuntu 5.15.0-18.18-generic 5.15.12
Uname: Linux 5.15.0-18-generic x86_64
ApportVersion: 2.20.11-0ubuntu76
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: MATE
Date: Fri Feb 11 10:36:05 2022
InstallationDate: Installed on 2022-02-06 (4 days ago)
InstallationMedia: Ubuntu-MATE 22.04 LTS "Jammy Jellyfish" - Alpha amd64 (20220206)
SourcePackage: mate-panel
UpgradeStatus: No upgrade log present (probably fresh install)