Added the possibility for defining the menu structure and contents through apps.

docs/howto/plugins/apps.md

...rial/nomad-countries/src/nomad_countries/apps/__init__.py

examples/data/rdm_tutorial/tutorial.ipynb

gui/src/components/UserdataPage.js

gui/src/components/UserdataPage.spec.js

gui/src/components/dataset/DatasetPage.js

gui/src/components/editQuantity/QueryEditQuantity.js

gui/src/components/entry/properties/SampleHistoryCard.js

gui/src/components/nav/Routes.js

gui/src/components/plotting/PlotHistogram.js

gui/src/components/plotting/PlotScatter.js

gui/src/components/plotting/common.js

gui/src/components/search/Filter.js

gui/src/components/search/FilterRegistry.js

gui/src/components/search/FilterRegistry.spec.js

gui/src/components/search/FilterTitle.js

gui/src/components/search/Query.spec.js

gui/src/components/search/SearchBar.spec.js

gui/src/components/search/SearchContext.js

gui/src/components/search/SearchContext.spec.js