Add some tests for NotebookTab (#5070)

* EmptyApplication: Add asserts to rest of getters (except for getSeventvAPI)

* Theme: make getTheme call getIApp()->getThemes() instead

this allows it to be used in tests
realistically this should be deprecated & users of it should just call
getIApp()->getThemes() directly

* Use getIApp() instead of getApp() in a few places

tests/CMakeLists.txt

@@ -36,6 +36,7 @@ set(test_SOURCES
     ${CMAKE_CURRENT_LIST_DIR}/src/XDGDesktopFile.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/XDGHelper.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/Selection.cpp
+    ${CMAKE_CURRENT_LIST_DIR}/src/NotebookTab.cpp
     # Add your new file above this line!
     )