[Composer] Fix The ComposerMap's tooltip is not updated after readXML

The ComposerMap's tooltip is only set in the private init method. This method is only used in the ComposerMap constructor. The result is that the tooltip is not updated after reading the id from the XML.

The solution could be to add a private method updateToolTip used when the id is changed :
* constructor
* assignFreeId
* readXML
[Composer] Fix ComposerMapWidget's label is not explicit

The user can only know the ComposerMap's label with it's tooltip. This commit
 proposes to update the ComposerMapWidget'label to be more explicit and to be
 the same as the ComposerMap's tooltip.