VfmdLine: It's not right to create/cast a VfmdLine from a VfmdByteArray
VfmdByteArray: Trimming a bytearray
VfmdLine: Make it a hard condition that lines must end with a '\n'
at creation time
Block handlers shouldn't have to create a VfmdLine from a VfmdByteArray

They should modify the line they get instead of creating a new VfmdLine instance.
Move the code for registering default elements out of the "core/" folder
VfmdByteArray: Locating a byte in a bytearray
Fix VfmdByteArray::print()
Optimize VfmdLineArray::clear()
Fix comment for description() of a block handler
Pass current and next lines to the block handler

Instead of the block handler having to call currentLine() and nextLine()
on the VfmdLineSequence.
VfmdByteArray: Convenience method isInvalid() (equal to !isValid())
Pass current and next lines to the block sequence

The block sequence can infer the end of the parent line sequence by
checking if the next line is valid or not.
Refactor RefResolutionBlockLineSequence for clarity
VfmdInputLineSequence: Remove ability to query current/next lines

We always pass the current/next lines to the block handler, so
there's no need to be able to query this.
We should be able to ask each block line sequence what block element it represents
Make the VfmdInputLineSequence keep a weak reference to it's parent VfmdBlockLineSequence

This would enable us to query the complete ancestor chain of blocks when
processing nested blocks.
Paragraph: Use VfmdBlockLineSequence::elementType() to determine the container type
There's no need to setContainingBlockSequenceType() on a VfmdInputLineSequence

The equivalent functionality is offered by:
    vfmdInputLineSequence->parentLineSequence()->elementType()
Refactor VfmdInputLineSequence: Pass around current and next lines
Refactor VfmdInputLineSequence: Avoid keeping track of m_currentLine and m_isAtEnd in member variables