Implemented support for modifying the text appearance stream text

By looking at how Adobe treats it's PDF FormFieldText with Format Actions, I could realize that the text displayed by rendering, is always on the AP stream. As of now, we only draw from this AP stream for the first time we open the PDF. By implementing these changes to Poppler, we can modify this at will, since the AP stream is generated every time drawFormFieldText is called.

Then, we can have the internal text saved on the \V, which will be the text used for all operations, and the text on the Appearance Stream, which is responsible for our hard drawing. This allows for interoperability between Adobe and Okular forms.

I couldn't make it able to read from this AP stream on opening.