Studio: avoid lambda functions for signal callbacks

[rodolforg]

Specially important for WorkArea queuing rendering.

Reasoning by Kjell Ahlstedt (gtkmm maintainer) follows below:

From the documentation of sigc::mem_fun():

 * @note If the object type inherits from sigc::trackable, and the
 * functor returned from mem_fun() is assigned to a sigc::slot, the functor
 * will be automatically cleared when the object goes out of scope. Invoking
 * that slot will then have no effect and will not try to use the destroyed
 * instance.

In most examples in the gtkmm tutorial the signal handlers are members of widgets.
All widgets inherit from sigc::trackable. The automatic disconnection when a
widget is destroyed would be lost if a sigc::mem_fun() is replaced by a C++ lambda
expression. Unless it's combined with sigc::track_obj():

m_button.signal_clicked().connect(sigc::track_obj([this]{ on_button_clicked(); }, *this));

but that's no better than sigc::mem_fun(), is it?

This kind of automatic disconnection is not always necessary. Some instances of
mem_fun() could be replaced by lambda expressions, to show that it's possible.
Preferably together with a description of the pros and cons of mem_fun() and
lambda expressions. But I don't recommend replacing the majority of the mem_fun()s.

https://gitlab.gnome.org/GNOME/gtkmm-documentation/-/issues/10

[rodolforg]

The changes in WorkArea can fix https://forums.synfig.org/t/synfig-dev-v1-5-0-is-crashing/12480 .

A while ago I had some crashes when changing the selected duck (or when unselecting them all) and I tracked down to this problem. I forgot to make a PR.

[ice0]

Merged. Thank you!