Classical tabs are very dated. They are also a practical problem as you
get very limited in the numbers of tabs we can have, and how long the
text can be on them.

Switch to one popular modern model with a list on the left instead where
pages can be selected.

The accessor functions are called value() for all standard FLTK widgets,
so let's keep that theme here as well.

Follow upstream FLTK naming to more clearly indicate that this is a
general widget and not TigerVNC specific.

These are general things and not specific to TigerVNC, so let's move it
to the fltk specific directory for clarity.

These have nothing to do with layout, so let's split them to their own
file.

Let's separate TigerVNC specific things from stuff that could be part of
upstream FLTK. These are files that we would like to collaborate with
other FLTK users, so they are more liberally licensed and avoid using
TigerVNC specific things.