A common pattern when using libtextscreen is to want to show or hide
particular UI components depending on the values of particular
variables. It is also common to want to only add things to the UI if
a particular boolean condition is true. Add a new conditional widget
to cover the first case, and a convenience TXT_If() function for the
latter.
This change rewrites and simplifies the copyright headers at the top
of all source files:
* Remove "Emacs style mode select" line; this line was included in
the headers for the originally released source files and appears
to be to set the file type for old versions of Emacs. I'm not sure
entirely why it was required but I don't think it is any more.
* Remove "You should have received a copy of..." text from copyright
header. This refers to the old 59 Temple Place address where the
FSF headquarters used to be located and is no longer correct.
Rather than change to the new address, just remove the paragraph
as it is superfluous anyway. This fixes#311.
* Remove ---- separator lines so that the file headers are barer
and more simplified.
