Added the ability to get a list of existing groups and a list of sections for each group.

Based on the request #143. In V7Preview4, it became possible to get a list of all groups, as well as a list of sections for any group, not just the current one.

New methods added:

• GetTerminalsGroups() - returns all existing groups
• GetRestaurantSectionsByTerminalsGroup(ITerminalsGroup terminalsGroup) - returns sections of the specified group in the arguments

In addition, the section now has a link to the parent group: IRestaurantSection.TerminalsGroup

The method for getting sections of the current group GetRestaurantSections will not be available in V7Preview4. Instead, you can use

// Get all sections of the current group
var currentGroupSections = PluginContext.Operations.GetRestaurantSectionsByTerminalsGroup(PluginContext.Operations.GetHostTerminalsGroup());