Make it possible to build without Qalculate

This allows me to build without the Qalculate dependency which is only
used for the calculator runner. Not having Qalculate installed will still
be an error unless the user explicitly passes
`-DCMAKE_DISABLE_FIND_PACKAGE_Qalculate=True`.

CMakeLists.txt

@@ -71,9 +71,10 @@ set_package_properties(KF5Baloo PROPERTIES DESCRIPTION "File Searching"
                        TYPE RECOMMENDED
                        PURPOSE "Needed for the File Search runner."
                       )
-find_package(Qalculate REQUIRED)
+find_package(Qalculate)
 set_package_properties(Qalculate PROPERTIES DESCRIPTION "Qalculate Library"
                        URL "https://qalculate.github.io/"
+                       TYPE REQUIRED
                        PURPOSE "Needed for precise computation in the calculator runner."
                       )
 

runners/CMakeLists.txt

@@ -2,7 +2,9 @@ if(KF5Baloo_FOUND)
  add_subdirectory(baloo)
 endif()
 add_subdirectory(bookmarks)
-add_subdirectory(calculator)
+if (QALCULATE_FOUND)
+    add_subdirectory(calculator)
+endif()
 add_subdirectory(helprunner)
 add_subdirectory(locations)
 add_subdirectory(places)