Add GUI for searching and loading basemaps

[giswqs]

This feature will add a GUI for searching and loading basemaps from xyzservices and Quick Map Services (QMS).

References:

• geopandas/xyzservices#65
• https://xyzservices.readthedocs.io/en/stable/api.html

[giswqs]

This feature has been implemented. See notebook example https://leafmap.org/notebooks/31_search_basemaps