This branch is where I (jmarc) try to implement string_wise metrics
computation. This is done through a series of cleanups. The expected
speed improvement will only be visible at the end of the road: indeed
for now we intend to keep unchanged behavior for testing purposes.

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

* change breakRow operation to operate on text strings on which
  metrics are computed. Note that for now
  FontMetrics::width(docstring) still computes the sum of character
  widths, so that behavior is unchanged.

* Implement proper string metrics computation (with cache), when
  lyxrc.force_paint_single_char is false.

Next steps:

* Make breakRow build a list of elements (string, inset,
  separator,...) in the row. This will be reused by other methods

* get rid of rowWidth (breakRow does compute this)

* re-implement getColumnNearX using row elements

* re-implement cursorX using row elements

* re-implement row painting using row elements (can it be done?)

* profile and see how performance can be improved.

Difference in behavior
* words longer than the screen are no monger broken at an arbitrary
point. This will not be useful anymore with horizontal scrolling.
* end of paragraph markers metrics are computed with the font of the
actual text, not default font. This will be extended to the other
methods.

The other differences should be considered as bugs.