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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.

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.

* Get rid of lyxrc.force_paint_single_char. This was 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 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.

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