Do not use "due today" color for completed to-dos in Agenda View

Do not use the "due today" color for the background color of a to-do in
the Agenda View if the to-do has been completed.  This is consistent with
to-do colors in the Month View.

BUG: 122776
1 job for release/21.08 in 2 minutes and 14 seconds (queued for 46 minutes and 3 seconds)
Status Name Job ID Coverage
failed build_clazy_clang_tidy #131875


Name Stage Failure
build_clazy_clang_tidy Build
1 warning and 3 errors generated.
make[2]: *** [src/CMakeFiles/KF5EventViews.dir/build.make:229: src/CMakeFiles/KF5EventViews.dir/agenda/decorationlabel.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:217: src/CMakeFiles/KF5EventViews.dir/all] Error 2
make: *** [Makefile:146: all] Error 2
Uploading artifacts for failed job
Uploading artifacts...
WARNING: JUnitTestResults.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1