Add qgis3 support

[Taifin]

This pull request consists of all changes necessary to run the plugin in Qgis version 3.x.

Changes include:

1. Migration of python2 to python3 using 2to3 tool
2. Migration of qt4 to qt5 with manual/automated refactorings
3. Migration of qgis2 to qgis3 apis with qgis2to3, qgis2apifinder tools and manual/automated refactorings

Some of the steps are described in this guide.

Compatibility with qgis3 was tested manually and all core features should work on newer versions.

[eox-cs1]

Thanks Taifin for submitting this transformation.
I will do some thorough testing and come back to you
ciao

[misev]

I just checked it now and it works well on Ubuntu 22.04, e.g. with endpoint https://ows.rasdaman.org/rasdaman/ows?
It would be helpful if you could clarify what didn't work for you @eox-cs1?