Add button for holiday plugin to reload holiday definitions

[ckunki]

In case user adds / removes / updates some (new) holiday definitions to/from/in configuration file and wants to check whether WR displays holiday instances correctly.

[kaklakariada]

I think most users will get a predefined configuration file and don't need a feature like this. Users who are debugging the holiday configuration can restart WR. I agree that this is inconvenient but I would like to avoid adding features that are rarely used and must be maintained.