Treat all \xFF strings as internal (also user)

This allows user code to create internal properties too, so that the
internal properties are not returned by e.g. getOwnPropertyNames().

Prior to this change, a user string beginning with \xFF would not be
treated as internal - unless it happened to be a string already used
by Duktape for its own purposes.  This is quite confusing.
Rename propertyvirtualization file
Initial draft of internal properties in guide
Release note: internal strings
Testcases illustrating access to Date _value
Testcase for user internal props

Also tag Date _value access test case as custom.
Clarify Duktape internal property name convention
Add an example of C string hex escape quirks
Internal property hex escape suggestion in guide
Internal property test: invalid utf-8 but no 0xFF

If a string does not begin with 0xFF it won't be treated as an internal
string even if it is invalid UTF-8.
API testcase for internal properties for user code
Hobject design doc: internal prop update, cleanup

Update discussion of internal properties, fix a few markup bugs, and
update "e_used" references to "e_next".
Merge branch 'ff-prefix-internal'