Sign Cate installers and apps

[forman]

Expected behavior

Cate installers and apps should be signed by appropriate certificates.

Signed installers are a prerequisite for auto-updates (see also #404, #465).

Actual behavior

Windows and Mac OS users will see a warning when they install Cate. On Mac OS user must even allow application to be installed from anywhere:

On older Macs:

Steps to reproduce the problem

Install Cate on Windows or Mac OS.

Specifications

Cate 1.0

[forman]

Done on build servers