The engine has been rewritten from scratch successfully, and is now more stable and clean than ever!
Back on the level editor, I’m working on the collisions engine. At first we decided to use a vector-based collision engine, but today we felt that pixel based collisions are enough. The former was too much work to get it functional and the latter is more precise, especially seeing we’re basicly (re)creating a pixel based game.
Optional default collisions for a tileset are now stored on the filesystem as image as a black-white mask, containing tiled collisions. The full-solid tile and blank tiles are hardcoded, so this way collision masks are optional. But they can be used to add stuff like slopes, half-tiles, etc to the collision maps. :)

Here’s a screenshot. Top right image is the tileset, bottom right image is the optional tileset collision mask, and the application itself (top left) shows all the unique tiles in the tileset collision mask. ^^ I know a few tiles are incorrect on that mask (slopes, for example), but I’ll fix that later.
Now to add a substate in the Level Editor to actually view, place & remove them (collisions per tile), and to save and load them. After that I can start working on the basic player physics with pixel collisions! Rawr.