Change property visibility to allow more flexibility for apps

[lubosdz]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	introduced by #18975, fixes #19696

Commit #18975 throws warning when reusing the View object more than once in a single request. This unfortunately ignores possible legit use cases. For example in one our app we have CMS admin, which requires in first step fetching & parsing View output from the page being edited (find placeholders) and then in returned view (2. View usage) display editable areas. Since 2.0.44 our logger is bloated with warnings You\'re trying to register a file after View::endBody() has been called. Since the property has been assigned private, there is no way to allow overriding behavior in an application e.g. for specific controllers only.

This commit alleviates aforementioned issue by changing property visibility from private to protected.

[what-the-diff]

• The private property $_isPageEnded is changed to protected.
• A warning message has been added in the registerFile() method when trying to add a file after View::endPage().

[samdark]

Need a line for CHANGELOG.