[2.3] [Form] Renamed option "virtual" to "inherit_data" and improved handling of such forms

Bug fix: yes
Feature addition: yes
Backwards compatibility break: yes
Symfony2 tests pass: yes
Fixes the following tickets: #5899, #5720, #5578
Todo: -
License of the code: MIT
Documentation PR: symfony/symfony-docs#2107

This PR renames the option "virtual" to "inherit_data" for more clarity (the old option is deprecated and usable until 2.3). It also fixes the behavior of forms having that option set.

Forms with that option set will now correctly return their parents' data from `getData()`, `getNormData()` and `getViewData()`. Furthermore, `getPropertyPath()` was fixed for forms that inherit their parent data.