#5 Keyboard shortcuts to write over null heads

[briareus]

Right now, null heads can only be overwritten by mouse-dragging another structure on top of them (generally from the list on the left, when constructing a tree). This is both time-consuming and potentially problematic (in combination with the bottom-of-the-screen bug). Right now, when a keyboard shortcut for inserting a structure is used at the mother of a null node, that structure can only be inserted as a sister to the null node; would it be possible to include the null node itself in the list of possible positions, and have the new structure overwrite it when that option is selected?