Improve Pixellate filter handling of selections
Start work on selection tool handling off-canvas locations
New directory structure (PortableApps.com format)

The new format separates content into two folders:

APP/ - essential files universal to all PhotoDemon installs (core
plugins, language files)

DATA/ - non-essential files unique to each PhotoDemon install (e.g. user
data)

This is not only a better semantic organization, as the user directory
can be freely deleted without consequence, but it was also the biggest
hurdle to making PhotoDemon PortableApps compliant.