New UI elements -- checkboxes, text input, horizontal sliders

[bwhitman]

I've been having a lot of fun building the forthcoming Tulip Synth Editor (on Tulip itself!) but realized we needed a few more UI elements to really make it useful. So I've added checkboxes, horizontal sliders, and text entry boxes. You tap on a text entry box and the Tulip OS will divert key presses to the box until you tap again or press enter. I've updated the buttons.py example to show them off.

We also now have support for deactivating UI elements. This is useful for multi page applications where you want to keep state of UI elements that are "hidden."