Open the config window in the centre of the app

Also:

• Opens the mobile settings page when the configure shortcut is pressed
• display a readable error if the config window fails to open

Test plan

Open the config window either from the menu bar or from keyboard shortcut.

• Previously: would open in the centre of the screen (or whatever the window manager is configured to do for new windows I guess), even if the main window is placed somewhere else
• Now: opens in the centre of the main window

Launch elisa in mobile mode & open the config page from keyboard shortcut

• Previously: opens the desktop config window
• Now: opens the mobile settings page

Modify the ElisaConfigurationDialog.qml by adding the line badProperty: false, launch elisa, open the config window

• Previously: error message in console is just "QQmlComponent: Component is not ready"
• Now: the error message also says "qml: qrc:/qml/ElisaConfigurationDialog.qml:21 Cannot assign to non-existent property "badProperty""

Bugs fixed

BUG: 429936