The $_level will be increased by 1 for each
If there's no $_last element (which is set on a newLine), we do nothing
(because there's nothing to indent into)
The $_last node is set as the $_currentParent node and acts as a parent-node
for further created nodes (They get appended in handleNewLine)
import-nodes can't be indented into, because they can't have children (poor imports :'( )
The opposite of this is, obviously, handleOutdent with -tokens