Implemented ComposerItem's Uuids

- uuid are kept on cut/paste, but deleted on import template or copy/paste
- removed obsolete id reference in QgsComposerLabelWidget
ComposerMap now uses the normal ComposerItem Id's implementation
(so the ID is now a QString instead of an int)

---------------
QString id() [inherited from QgsComposerItem]
    instead of
int id()
---------------
void setId(QString id)
    instead of
void setId(int id)
---------------

- also fix all dependent composer items (table, legend, picture, scalebar, map)
Correct problems with map overview and make the problems with map overview group box checkable