Android app: build safe AsyncTask behaviour into a derived class
Remove use of uninialized context.

Harmless, since the context wasn't used, but confusing.
A few general utility functions added for the sake of the forms work
Forms API for native mode
Forms: handle field appearance change on mouse up/down
Forms: initial simple handling of check boxes and radio buttons
Forms: inhibit "wait" cursor when updating display just for a form change
Forms: part of text field handling
Forms: handle some cases of setting text-field text
Forms: create apearance streams for text fields when not present in the file
Forms: handle font size zero in default appearance
Forms: centre text vertically
Forms: add text input dialog to win32 app
Forms: initialise stream contents of freshly created text appearance streams

ensures that saving before the user fills out the field will put sensible contents in the file.
Forms: handle scaling text to fit
Forms: update field value on text entry
Forms: fix the base line for fit to width text
Forms: use a text matrix to fit text rather than altering the font size

To achieve sufficient accuracy, non-integer font sizes were being used,
which MuPDF can handle, but may be illegal. The text matrix is the better
method.
Forms: replace the marked content rather than the entire stream
First go at Javascript-engine abstract API