Checks are only about code linting, which isn't interesting for us

- Bump to [new upstream release](https://github.com/mate-desktop/engrampa/releases/tag/v1.28.2)
- 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- Remove outdated patch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

FTR, commands used to change arch=() and then verify the changes:

$ sed -Ei "/^arch=/{s/ *?['\"]?i686['\"]? *?//;s/ *$//}" */*/PKGBUILD*

$ grep -r i686 * | grep arch= | grep -v repos

$ svn diff | grep -A 1 ^-[^-] | grep -v -- -- | cut -c2- |
  while IFS=$'\n' read -r l1; IFS=$'\n' read -r l2; do
    echo "$l1 -> $l2"
  done | sort -u