Continue to overhaul spec file

[Olf0]

Continue with tasks from MR #133, comment #8:

• BuildRequires: PackageKit-Qt5-devel in line 27 → BuildRequires: pkgconfig(packagekitqt5)
• %defattr(-,root,root,-) → (?), see second point of the original message here for details.
• %if 0%{?sailfishos_version} && … → %if %{defined sailfishos_version} && … (and at other locations), see this message here for details.

[Olf0]

1. The first bullet point was addressed in commit f063d79.
2. The second bullet point still needs to be evaluated before being applied to "production software" and was not yet part of this PR

3. The third bullet point was addressed in commit f195a6c: Use defined macro for testing if a macro exists 0%{?sailfishos_version:1} would have been equivalent in this case, but defined as already present in SFOS 2.2.1 and presumably even much earlier.

   Useful references for RPM conditionals (there are many not useful ones):

   • https://backreference.org/2011/09/17/some-tips-on-rpm-conditional-macros/index.html
   • https://rpm-packaging-guide.github.io/#rpm-conditionals
   • Only for its explanations: https://en.opensuse.org/openSUSE:RPM_conditional_builds#Conditional_Variants
4. The remaining changes are better not applied, because it reduces build_yaml to being tested for existence instead of being set to 1, which is the proper test.