Add support of default_basemap for themes

Related to https://github.com/camptocamp/c2cgeoportal/pull/981
In all other plugins having a "layerTreeId" param, the default value is null. For instance:
https://github.com/camptocamp/cgxp/blob/master/core/src/script/CGXP/plugins/ThemeSelector.js#L53
https://github.com/camptocamp/cgxp/blob/master/core/src/script/CGXP/plugins/FullTextSearch.js#L108