Add contextual reminder for Core Settings → Highlights

Test case

Steps

1. Connect to a Quassel core 0.12.5
2. Dismiss the warning dialog
3. Open ... menu → Core Settings → Highlights

Expected

Some warning about missing support for core-side highlights.

Example based on the buffer activity warning:

Quasseldroid cannot configure highlights. Upgrade your core to Quassel v0.13 to resolve this.

Actual

No message about missing features, no sign that it doesn't work, except for changes not saving.

Additional

This is part of the work on contextual feature reminders, #93, prioritized based on how often highlighting on nicknames is a requested feature.

It probably would still complement a missing features banner in Core Info, #172.