Using a point-and-click adventure studio for a retro-looking platformer ... well, that's unexpected, but the result sure looks good and doesn't seem to have quirks in gameplay, so congrats 
I demonstrate the way I implemented movement, acceleration/deceleration on different surfaces (solid and slippery)
It's good you took some time on this. Imho, that's a key point to make a game feel professional opposed to kiddies programming experiments. I wonder whether you encountered the same kind of update animation only when virtual position of the character is n pixels away from current position as the one I used in my engine.
Did I correctly picked that you map dash as DOWN+L/R+JUMP ... do you have any reason to avoid DOWN+ATTACK ?
Thanks PypeBros!
I didn't quite understand the problem(?) - my French isn't very good and the English explanations didn't really help me understand very much. But I had very little problems implementing the ice surfaces: I have a friction variable that is simply altered, depending on whether or not the player is currently on ice. And friction simply affects the speed of acceleration/deceleration. The animation of the player is played depending on the keys that are pressed and synced so they look good (animation speed vs movement speed) on standard, non-slippery surfaces. On ice they do look like 'moon-walking' while you're slowly picking up speed but that is, of course, the effect we're after, since the surface is so slippery and makes you 'moon-walk' in real life.
About the dash-move: you understood correctly, it's DOWN+L/R+JUMP. As to why I didn't do DOWN+ATTACK? Well, I feel adding in a direction key gives the player more control and speed - which, once again, is what I'm going for with this game, I DON'T want ANY kind of slow and 'rigid' controls, it's all about fast, manic and, hopefully, fun movement. So in the game, you don't have to face the right direction first, then hit DOWN+ATTACK, you can just hit DOWN+JUMP with L or R and immediately start dashing in that direction. Even when you're at full speed, running right, you can still immediately hit DOWN+L+JUMP and change your direction on the spot. This isn't physically accurate (much like double-jumping for example) but, to me, it's fun that way.