Create annotation tool to add a median point while creating elements

A new tool should be added to help create complex shapes in the following workflow:

• a user wants to create a text line that has some slope
• he creates a rectangle
• then adds median points so he can quickly tweak it

Let's add a new tool in the image editor that adds a point in-between two points:

• when the tool is selected
• the user click on a line
• a new point appear at the median point between these 2 points