It also add some properties such as `border_width`, `border_color`
and `preferred_alignments`.
It also fix a documentation bug where the `margin_topleft` was called
`margins_topleft`. To conform to the documentation, both are now valid
but one should be removed the next time the API changes.
Fixes#1978
The requirement to call add_signal() was added to catch typos. However, this
requirement became increasingly annoying with property::<name> signals and e.g.
gears.object allowing arbitrary properties to be changed.
All of this ended up in a single commit because tests/examples fails if I first
let add_signal() emit a deprecation warning.
Signed-off-by: Uli Schlachter <psychon@znc.in>