!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
PLEASE DO NOT DO WORK ON TOP OF THIS BRANCH.
I INTEND TO REWRITE HISTORY LATER!!!
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This branch is where I (jmarc) try to implement string-wise metrics
computation. The goal is to have both good metrics computation (and
font with proper kerning and ligatures) and better performance than
what we have with force_paint_single_char. Moreover there has been
some code factorization in TextMetrics, where the same row-breaking
algorithm was basically implemented 3 times.

Currently everything is supposed to work for both LTR and RTL text.

The bugs fixed and caused by this branch are tracked at ticket #9003:
http://www.lyx.org/trac/ticket/9003


What is done:

* Make TextMetrics methods operate on Row objects: breakRow and
  setRowHeight instead of rowBreakPoint and rowHeight.

* Change breakRow operation to operate at strings level to compute
  metrics The list of elements is stored in the row object in visual
  ordering, not logical. This will eventually allow to get rid of the
  Bidi class.

* rename getColumnNearX to getPosNearX (and change code accordingly).
  It does not make sense to return a position relative to the start of
  row, since nobody needs this.

* Re-implement cursorX and getPosNearX using row elements.

* Do not honor lyxrc.force_paint_single_char anymore. This is a
  workaround that is not necessary anymore.

* Implement proper string metrics computation (with cache). Remove
  useless workarounds which disable kerning and ligatures.

* Draw also RtL text string-wise. This speeds-up drawing.

* Do not cut strings at separators in RowPainter when text is not
  justified. This speeds-up painting by reducing the number of strings
  to draw.

* Do not cut strings at selection boundary in RowPainter. This avoids
  ligature/kerning breaking in latin text, and bad rendering problems
  in Arabic.

* Remove homebrew Arabic and Hebrew support from Encoding.cpp. We now
  rely on Qt to do handle complex scripts.


Next steps:

* Get rid of LyXRC::force_paint_single_char, which is not used anymore
  in the source.

* Maybe get rid of LyXRC::rtl_support, which does not have a real use case.

* Fix bugs uncovered by testing.

* Profile and see how performance can be improved.


Steps for later (aka out of the scope of this branch):

* Re-implement row painting using row elements. This is not difficult
  in principle, but the code is intricate and needs some careful
  analysis. The first thing that needs to be done is to break row
  elements with the same criteria. Currently breakRow does not
  consider on-the-fly spell-checking, but it is not clear to me that
  it is required. Moreover, this thing would only work if we are sure
  that the Row object is up-to-date when drawing happens. This depends
  on the update machinery.

  This would allow to get rid of the Bidi.cpp code.

* Change Row object to operate only on integers and not doubles. Then,
  rewrite the computation of spacing in justified paragraphs so that
  strings can be drawn without cutting at separators. This will improve
  performance.