* Even when emitting smcup, emit an explicit screen clear, to handle
smcup that fills the screen with old background color #1045
* Rather than emitting notcurses_mouse_disable(), emit the more
powerful (proper superset) reset_term_attributes)
* Emit a diagnostic when we're not connected to a terminal
Fixes: https://github.com/dankamongmen/notcurses/issues/1009
Whenever a widget is created with its `*_create` function it currently
claims full ownership of the passed panel, including its destruction.
However, the C++ wrapper around the panel is not aware of this and will
attempt to destroy the native panel in the destructor, leading to
segfaults.
Fix this by introduction of a `Widget` class which contains the logic to
properly modify the `Panel` instance to not double-destroy the native
panel. The solution is a bit fragile since the `Panel` instance is left
intact (we can't free it for the user) in a state that's safe for the
C++ wrapper, but calling any C function via the wrapper **will** pass a
`NULL` pointer in the panel argument - therefore the C functions MUST be
proofed against this. The proofing belongs in the C backend code since
this protects also C and other language binding users from such abuse.
The Widget class will first verify that the passed `Plane` instance
hasn't already been "disowned" and will throw an exception to the effect
if it was. Next, it will proceed to take over ownership of the native
panel instance and mark the passed `Panel` as "invalid" (i.e. not owning
any native panel instance anymore)
The above changes require modification of `Panel` instances and so all
the widget constructors taking `const*` or `const&` have been removed
from widget classes.
