Disable thumbnails when only single account logged in.

[feyyd]

Based off master branch, this feature disables thumbnails when only a single account is logged in.

Added an option to the main settings to enable/disable, default is disabled.

Currently a couple questions but should be easily resolved:

1. How to handle accounts that are on character select screen, currently I am counting this as "logged in"
2. Doing the check every tick and setting the enable/disable flag on each thumbnail. I didn't add a temp bool so we run only if changed, currently it just iterates through each thumbnail and sets the enable disable flag. Its performing this flag change every 500ms per character and only toggling a bool so I didn't overengineer it here.
3. Please double check my option, this is the first time I've mucked with the UI, but I basically just copied how the other options were implemented.