This PR contains all from the new settings management. Let's explain a bit how it works :

• init.lua : here you defines the settings
  • mail.settings : the whole list of settings with setting = { -- data } structure
  • mail.settings_groups : the list of groups of settings (for settings page)
  • then it generates selected_idxs sub-tables for each settings
• settings.lua : the redesigned ui page
  • first of all, it defines a base of x and y values
  • after that, it creates a temporary table for ordering settings depending on their index value (for the selected group)
  • then it iterates through the settings and generate UI for each type of settings
  • when the player sends fields, it saves in selected_idxs the temporary value

Info for settings definition :

• required : type (supported : string, index (for dropdowns), bool (for checkboxes)), default, group, label
• additional :
  • index (recommended) : used to defines the order of the settings in the group on the UI
  • tooltip : additional info at hovering
  • dataset :
    • required for index types (dropdown), for example default sorting field (From/To, ...)
    • using for giving predefined values for string (show a dropdown alongside)
  • format : for dataset, preview predefined values (for example format current date with %X ...)

Hope there's not too much bugs, I fixed all those I noticed. I'm surprised that it not contains too much new lines, and new settings implementation is now much easier.

And, as usual, a small screenshot :