Bump to 1.28.2 + Overall rework of the PKGBUILD

Changes:

• Bump to new upstream release
• Update pkgdesc
• Update license + use SPDX identifier
• Add missing glib2-devel make dependency (now required to build the package successfully).
• Use a more transparent source (clone of the upstream repo against the $pkgver tag). This implies the use of git submodules, additional make dependencies and extra steps in prepare() function (all included in that commit).
• Remove outdated patch
• Add check()
• Add nvchecker integration for the package

As you can see, I also added myself as a co-maintainer for the package, as I'm using it myself.

To be honest, I'm not necessarily interested in maintaining the whole mate packages stack (as I'm only specifically using engrampa on my side), but I'm fine giving some help to apply some of the major above changes to the whole mate package bundle, e.g. the license update + spdx indentifier, the switch to a more transparent source (including the usage of git submodules and prepare() function + extra make dependencies where needed), the addition of the check() function where missing and the addition of the nvchecker integration for all of them.

Let me know your thought 😄
I remain available if needed!