There were three principal sources of errors and warnings:
1. Un-ported `onFooChanged` statements
2. `config.fontSize` being unconditionally referenced from components
   without access to it (it comes from the SDDM config)
3. Accessing units from the context property rather than the singleton

All are fixed now. Tested with:
1. `sddm-greeter --test-mode --theme ~/kde/usr/share/sddm/themes/breeze/`
2. `~/kde/usr/lib64/libexec/kscreenlocker_greet --testing --theme ~/kde/src/plasma-workspace/lookandfeel`
3. `/kde/usr/lib64/libexec/ksmserver-logout-greeter --fakearg`

No regressions found.

BUG: 425349
FIXED-IN: 5.20