Gracefully handle terminal resize and font resize

[GitSquared]

Describe the bug Resizing my terminal's font or resizing the window breaks the presentation. It seems to me these are common enough adjustments for warranting an issue.

Steps to reproduce the bug Steps used to install present:

1. pip install present

Steps to reproduce the behavior:

1. Create a basic slide
2. Open it with present
3. Resize your font using terminal emulator shortcuts, or resize the window
4. Aaaand it's broken :(

Expected behavior

• Resizing the window or changing the font size triggers a repaint in Present, and the slides automatically adjusts.

Slides (not relevant)

Screenshots

Resizing demo	Changing font size

Environment

• OS: Linux (5.8.7-arch1-1 #1 SMP PREEMPT Sat, 05 Sep 2020 12:31:32 +0000 x86_64 GNU/Linux)
• Terminal emulator: Kitty
• Python version: 3.8.5
• present version: 0.5.1 (btw, would be nice to be able to do present --version

Additional context /

:v: