delta time cfg setting and smooth thing movement

[rainlizard]

My previous attempt broke multiplayer and had other issues so I started over and changed my approach to try to leave the logic alone and only alter the 'draw'. There's no smooth mouse or camera yet, they'll be a bit more troublesome this time.

For now we've got three aspects smoothed by delta time:

• Tooltips
• Liquid wibble
• Thing rendering positions

Tooltips and slab selector were flickering, so some lines had to be moved from draw to logic. Renamed gameplay_loop_sleep() to gameplay_loop_timestep(), in which it decides whether to go to sleep or use delta time.