Initial commit. Basic qt UI + custom QGraphicView

100% useless for now
Added mouse wheel zooming + mid button panning
Added an WIP 'letter selector' that tracks the mouse

+ sample snapshot saving by left click

The zooming is currently disabled since it interfere with letter
selector rotation
Added support to scale the letter selector (shift + wheel) and zoom (ctrl + wheel)

While here, added (currently useless) UI elements
Implemented first version of the 'letter saver'

Extract the zone covered by the letter selector and save it as PNG file.
Initial zoom factor to fit the whole page in the view
Save letters as g{x}-c{x}-{letter}.png

While here, connected letter selection to interface
Added spin boxes for groups and children
Added a 'Sheets' class to manage input sheets
Update to UI to change the current sheet
Changed the way the letter selector is placed on canvas

It now tracks the mouse as soon as the cursor enters the canvas