Fix bugs #8546 and #9055, and introduce new separator inset.

Fix bugs #8546 and #9055, and introduce new separator inset.

The algorithm used for breaking a paragraph in LaTeX export is changed
for avoiding spurious blank lines causing too much vertical space.
This change is tied to the introduction of a new inset (with two
different specializations) helping in either outputing LaTeX paragraph
breaks or separating environments in LyX. Both of the above goals were
previously achieved by the ---Separator--- layout and can now be
accomplished by the new inset in a more natural way. As an example,
after leaving an environment by hitting the Return key for two times,
a third return automatically inserts a parbreak inset, which is
equivalent to the old separator layout, i.e., it also introduces a
blank line in the output. If this blank line is not wanted, the
parbreak separator can be changed to a plain separator by a right
click of the mouse. Of course, an environment can still be separated
by the following one by using the Alt+P+Return shortcut (or the
corresponding menu key), but now the plain separator inset is used
instead of the old separator layout, such that no blank line occurs in
the LaTeX output.

Old documents are converted such that the LaTeX output remains unchanged.
As a result of this conversion, the old separator layout is replaced by
the new parbreak inset, which may also appear in places where the old
algorithm was introducing blank lines while the new one is not.
Note that not all blank lines were actually affecting the LaTeX output,
because a blank line is simply ignored by the TeX engine when it occurs
in the so called "vertical mode" (e.g., after an alignment environment).
The old ---Separator--- layout is now gone and old layout files using it
are also automatically converted.

Round trip conversions between old and new format should leave a document
unchanged. This means that the new behavior about paragraph breaking is
not "carried back" to the old format. Indeed, this would need introducing
special LaTeX commands in ERT that would accumulate in roundtrip
conversions, horribly cluttering the document. So, when converting a
modified document to old formats, the LaTeX output may slightly differ in
vertical spacing if the document is processed by an old version of LyX.
In other words, forward compatibility is guaranteed, but not backwards.