Plugin loading mechanism ported to Qt5/KF5

Now, Cantor is using K_PLUGIN_FACTORY_WITH_JSON and the
plugins are installed in different directories of Qt
plugin system:
    * backends are in cantor/backends;
    * assistants are in cantor/assistants;
    * panels are in cantor/panels;
The metadata now are using .json files and they are
inserted in plugin. .desktop files were deprecated
except by the libcantorlibs plugins - there are
.desktop files yet, but they are converted to .json
files during the build.

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