Fix bug with wrong baseline calculation in last paragraph

Fixes: #9231
Do not break documents with complex beamer block ERT usage we cannot handle

Fixes: #9144
Revert "Clear empty selections in GuiView after dispatch"

This reverts commit fb05011ad0db2e2c5e557d91f398970d75cf8d15,
which lead to various menu-related crashes, such as when
going to Help > About.
installer: update changelog
Do not allow copying a separator without a following par end.

This makes virtually impossible copying a separator inset whithout
also copying the end of paragraph. These insets are not supposed to
be directly inserted by users. For example, the parbreak version
represents a LaTeX paragraph break, not a LyX one. So, if it is
possible to copy and paste it by alone, an unsespecting user may be
surprised to see a paragraph break in the output but not on the LyX
screen (because of the lack of indentation, for example).
In this way, it also becomes a LyX par break from a user point of
view, not any more useful than simply introducing a par break by
hitting <return> (except in those cases where it makes a difference,
in which case they are automatically inserted by LyX).