Text annotations behavior differences

[dstillman]

Text annotations don't work the same as on the desktop:

1. The corner drag handles should only resize the box proportionally, and that should also resize the text. It shouldn't be possible to make the box narrower/wider or shorter/taller from those dots. This is creating some unreliability in resizing the text compared to the desktop app.
2. There shouldn't be drag handles at the top and bottom in the center, since it doesn't make sense to make the box less tall when the box size determines the font size.

(We might need to revisit these when we add explicit font size options, but for now, corner dragging is how you resize text, and that should work reliably.)