Is it possible to import MMF Projects? (mfa files...)
As I work already a long time with MMF, how difficult is it to switch to Construct?
That depends... some people seem to think the switch is easy, some people have a hard time of letting go of old klik habits (like spread values, which are totally not necessary in Construct). I found it to be pretty easy to switch over, personally.
There's also an Event Edtior etc...?
Yes, and it doesn't have any checkboxes! It reads top to bottom, like regular code, instead of like a spreadsheet in MMF, but you still click to create events in a similar manner. You can also reuse your event sheets on any layout (or frame... I believe that is the MMF term, it's been a while since I used it) so you don't have to copy/paste entire event sheets around.
You can also use more advanced eventing features like sub-events, many kinds of loops, and "real" programming type logic stuff like Or, Else, and functions. Or you could program parts of your game in Python if you want, and uh, if you know Python. But that's totally optional.
When I create a new Game on a 4:3 LCD, how will it look on a Widescreen LCD (16:9)?
Will it look "fat" stretched or is it possible to keep the screen ratio?
It will keep the screen ratio.