Two fixes in Context::get_default_interpreter

Two fixes in Context::get_default_interpreter


When creating a hierarchy above Bottom, all intermediate levels are named "\new"

Previously, get_default_interpreter gave all intermediate levels the
same id as that of the new Bottom level.

However, since issue 3225, there can be more than one Bottom in one
hierarchy since a Bottom context (defined by not having a
\defaultchild) can still have children contexts (of any type it
\accepts even while it would not create them implicitly).

So it is a bad idea to give implicitly created contexts the same id as
any explicitly named context.


get_default_interpreter should create contexts with missing context_id

At the current point of time, an existing bottom context is returned
even when there is a mismatch in ids.  For example, when writing

\context Voice = "one" <<
  { \voiceOne g' g' g' g' }
  \context Voice = "two" { \voiceTwo c' c' c' c' }
>>

creates two voices, while

\context Bottom = "one" <<
  { \voiceOne g' g' g' g' }
  \context Bottom = "two" { \voiceTwo c' c' c' c' }
>>

only uses a single Voice called "one".  This is arguably wrong.