ProjectExplorerSettingsPage: Do not have a const setter.

Change-Id: Ic0a71ce49b9f65894860562b8414772cd839152c
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
ProjectExplorerSettings: Keep environmentId.

Make sure to not change the environmentId when editing something
in the settings page.

Task-number: QTCREATORBUG-9359
Change-Id: I7fc891cb3a69f08b1c2ea3f963a2deb01ddca9b2
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>