RS 10 Seattle - Fix large fonts in dialogs (Update for solution)

An example for such a dialog is the Evaluate/Modify dialog.

Fix:

Change this in LargeFontFixRegisterUnit.pas :
- Application.OnModalBegin ==> Screen.OnActiveFormChange
And some code cosmetics :
- HookOnModalBegin ==> HookOnActiveFormChange
- FSavedApplOnModalBegin ==> FSavedOnActiveFormChange