pdTextBox: implement Enabled behavior

Still TODO are some UI bits when the control is disabled; the backcolor
of the DC is handled differently in that case, which I didn't know until
now!
pdComboBox: continued work

- Enabled property, same as pdTextBox
- Allow items to be added before the API combo box is created (e.g. at
Form_Load instead of Form_Show).  When the API combo box is finally
created, all previously added items will be automatically taken care of.