Added an open file dialogue for selecting a font

|SOURCE|
[MainWindow]
* Added an open file dialogue to the menu bar, this will be moved to the main
  UI section later instead of being obscured by the menu bar
Added a font rendering widget

|SOURCE|
[FontWidget]
* Renders the selected font on screen and allows them to be exported to a TGA
  (to follow)
* Still need to display lines indicating various metrics, such as the overhang,
  space between characters, and lines that encompass the face glyph
[MainWindow]
* Added a FontWidget instance, which is created when a font is selected from
  disk
* Currently, the FontWidget takes up the entire space of the window, this will
  be rectified later so it only shows up in a dedicated font viewing area