Limit the max height of image thumbnails

[kevinaboos]

This is unblocked now that we have two key features:

1. We have a full-screen ImageViewer widget now, so it's okay to show smaller thumbnails that don't clog up the room timeline view.
2. Makepad now supports Fit size values with either relative or absolute max values.
   • So we can do something like height: Fit { max: 400 } or whatever value looks good.

Optionally, we can add a setting in the SettingsScreen to allow the user to control the max size of thumbnails:

• no maximum
• a small maximum (like 200 px)
• a moderate max size (400 px or so)
• any custom value