Added a simple Targa exporter

|SOURCE|
[FontWidget]
* The sprite texture is written to a QImage instead of directly to the widget's
  drawing surface
* After the QImage is generated, a new QPainter object is created to allow for
  drawing the processed font to the widget
* The QImage is saved to a Targa file (hardcoded for now)
[Targa]
* WriteQImageToTarga takes a reference to a QImage and writes the data into the
  Targa file format with the path specified by the QString
* No error checking or colour checking is performed
Separated the overlay from the main image rendering

|SOURCE|
[FontWidget]
* When rendering the overlay, it is rendered to a separate QPainter and QImage
  than the main font texture renderer so as to allow for an easier separation
  of the font sprite sheet from the diagnostic overlay when verifying the font
  layout