Recode the core of the project

Code is divided in:
- photocollage.collage for Page, Column, Cell and Image object
- photocollage.render to create a PIL image from a Page
- photocollage.gtkgui to display the GUI

The GUI is also greatly enhanced.
Embed new icons for resize buttons
i18n: Update French translation
Filter user input for border width value
Add unit tests
Fix photos sometimes not displayed at bottom
Fix remove_empty_cols()
Fix error when cancelling high-res image saving
Show a dialog for asking poster dimensions when saving