I tried to search for the few questions, but couldn't find the answer... So, any idea why I cannot click the "Create brush" button to add new brush? Is this feature still to be implemented on Linux version or what?

Another problem conserns about the intersection of two lines. Lets say that I draw a circle. Then I draw freeformed curve, where start and end point are inside the circle, but the curve goes outside this circle. This way I get two intersection-points to circle. Now is there way to create intersection points to where these two lines (circle and freeformed line) collide? I'd like to cut out the part of circle between the curve, and then join intersection points of curve and the left-out part of circle then as one object...

Another example of this same problem: I draw two vertical lines, and one horizontal line which crosses both of these lines. Now I want to delete exactly the part which is between horizontal lines (from first intersection to another). How can I do this?