We ported units to use the PlasmaCore.Units singleton earlier; now it's
time to do the same thing for PlasmaCore.Theme.
There are no UI changes or regressions detected.
The context property version is slower to access and won't be supported
in Qt6. Let's port away from it and use the singleton version instead.
Here was my full process for making this change:
1. Made the change with `find . -name '*.qml' | xargs perl -pi -e 's/units\./PlasmaCore\.Units\./g'`
2. Verified no more occurrences with `grep -r " units."`
3. Made sure this didn't change any comments in a silly way by inspecting the output of `git diff | grep "+ " | grep "//"`
4. Manually inspected the full git diff to make sure there were no other unintentional or silly changes (there were none)
5. verified that all changed files have the PlasmaCore import with the correct name with `for FILE in `git status | grep modified | cut -d ":" -f 3`; do grep -q "as PlasmaCore" $FILE || echo "$FILE needs the PlasmaCore import"; done` (one needed the import)
