Plugins: File list: Always show expanders when option is turned on.
Plugins: File list: Set show_expanders properly.

When expanding directory with 'show_expanders' set to False program will
temporarily set this value to True for the current list only.
Plugins: File list: Improved readability.
Tools: Version check: Switched to GitHub API.
Read me: Fixed typo.
Read me: Changed gallery URL to new site.