Single input per page: Introduce item names

[RomanTsukanov]

There are many places in which we use generic names, like "item", "row", or "panel", by default. If users want to update these captions, they need to specify many properties.

We could just ask users to provide a localizable name of the items that a Dynamic Panel or Matrix displays, such as countries, orders, or products, and use this name instead of the generic names.

itemTypeName: "Country|Countries"

[SamMousa]

Note that some languages have multiple plural forms: https://en.wikipedia.org/wiki/Dual_(grammatical_number)