Closes #934

Closes #963

This is shown only when the maintainer mode is off. It should
be '#ifdef', instead of '#if', of course.

WebKit can wrap after the dash ('-'), similarly as before a space,
thus let the EConvert to wrap in the same way.

Thus it's better understandable in which place the error happened.
Also make the error message selectable for easier copying of it.

Related to #947

Closes #947

Better to use WebKit's own WEBKIT_FORCE_SANDBOX=0, than to duplicate
the same functionality.

This reverts commit 6959aeb5.

To avoid override of it in g_propagate_error(), which produces a runtime
warning, when there are set multiple servers and the former have failures.

Closes #821

Closes #925

Closes !57

Closes #906

Or better when it could change, according to GSettings. There is no need
to re-read the value whenever the message list content is regenerated,
especially because the option doesn't change that often. This also avoids
unnecessary memory allocations.

Closes #930