[FEATURE] prevent accidental use of parent setters

[wolframhaussig]

Is your feature request related to a problem? Please describe.
For the StylableTextBox, we added HintForeColor and TextForeColor as a replacement of ForeColor. Depending on the current value (empty => show hint), either HintForeColor or TextForeColor will be used for ForeColor. To prevent any accidental use, we already added Browsable(false) and EditorBrowsable(EditorBrowsableState.Never) to not show this property in the GUI editor. Unfortunately, this does not always help:

Control c = new StylableTextBox();
c.ForeColor = Color.Blue;

As the ForeColor is available for Control it can be used without any hint that there is any issue with that.

Describe the solution you'd like
We want to go for 2 different options:

• for Reflection, we will add a separate Attribute marking this setter for internal use only
• for callers, we will reuse our existing errorhandling and create an error when used directly