Edit Box - Plain Text Edit

Detecting line changes when selections are involved is turning out to be a rather involved.  In doing research on a number of things, I discovered that there is a QPlainTextEdit class, which is a much simpler version of the QTextEdit class, and is more appropriate for a code editor.  Namely, QTextEdit is more complex and can handle rich text (text with formatting, which was disabled for EditBox), lists and tables.

The QPlainTextEdit class only handles plain text, and will also handle larger files more efficiently.  Fortunately, this class will still support syntax highlighting and other highlighting that will be needed for EditBox including highlighting errors.

To switch EditBox to using this class as its base class, all that was needed was changing all instances of QTextEdit to QPlainTextEdit.  The call to setAcceptRichText() with a false argument was no longer needed.  And there is no such concept of a current font (QPlainTextEdit only has one font), so in order to set the font to the desired fixed width font, the font() and setFont() access functions inherited from QWidget are used to set the font of the edit box.

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