Add Haiku repository config only optionally

The HAIKU_ADD_HAIKU_REPOSITORY_CONFIG build variable can be set to add
the Haiku repository config to the image.
Paragraph: Cache length
MarkupParser: Make sure added paragraphs are valid

They need to contain at least one TextSpan, even if empty.
ParagraphLayout: Handle empty paragraphs.

Add one LineInfo for the empty TextSpan, if they have one. This gives the
layout a height for the one, empty line.
TextDocument: Don't leave empty paragraphs when inserting line-breaks.

Instead, add one empty TextSpan. This commit resolves the last of the
very obvious problems when editing. More subtle ones may still surface.