[kcms/keyboard] Use correct color for layout label in KCM

The label was always using the text color from the Plasma theme, which just happened to work fine as long you were using the same colors for both your Plasma and Apps colors (e.g. both dark or both light). But when using one dark and the other light, the text would become invisible.

This is resolved by adding a paremeter to the function that allows it to specify whether it should get its text color from the Plasma theme or apps color scheme. Then the function invocation for the tray item asks for the Plasma theme color and the invocation in the KCM asks for the apps color scheme color.

Now the label is always visible:

BUG: 431361 FIXED-IN: 5.21