Multiline text blocks

Support for multi-line text blocks

Adds a feature flag to enable multi-line text blocks (disableMultilineTextLayer=false), disabled by default.
The purpose is to make the DOM lighter and the text selection more natural, e.g. less flickering on text selection. Additionally added a flag to make the text layer visible (debugTextLayer). Mainly checked the code with
- the Trace Monkey document
- the NASA document (#4339)
- the PDF 1.7 spec
- some Russian document (#4626)
- and some French document (#4487)

Especially with the last two documents the scroll performance has improved remarkably, mainly due to concatenating of chars to words.

Also changed the highlight color to system color as a suggestion.