empty new lines in text boxes need to be displayed lower

on 3.0dev 6c36ccc (this glitch is not present on 2.1dev)

reproduction steps:

1. create new textbox (or double click on any existing textbox)
2. add a new line character, and observe that the cursor moves down sufficiently below the earlier text to be seen without overlapping
3. click outside of text box
4. double click back on text box
5. move cursor all the way to right or all the way down or press [End]
6. observe that the new line hasn't moved down sufficiently, and that the cursor is overlapping the previous line of text

Fortunately, once you start typing text, the newline will get enough vertical space.