Fix text view holding onto vim buffer

The changes for fixing #925 introduced failures in the memory leak integration tests because the fallback processor retains a reference to its corresponding vim buffer.

This change fixes those test failures by arranging for text view key processors that correspond with vim buffers to coordinate their actions directly with the fallback processor.  All other text view processors use the fallback key processor unconditionally.  As a result, the fallback key processor itself no longer needs a reference to the vim buffer.