Some frame.py typing, move seldom used methods in frame.py

[mroeschke]

Description

• _drop_na_columns was only use on IndexedFrame and not Frame so moved the method to `IndexedFrame
• _has_nulls is equivalent to isna().any()
• Some typing

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

@mroeschke there's a real issue here that's popping up in the Cython code.

[vyasr]

/merge