Fix all errors and warnings on login, lock, and logout screens

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