Drop libcaphe and rely on GtkApplication to inhibit suspend

The ultimate goal is to :

fix #117 (closed)
simplify the codebase, as I don't have time to properly maintain and test libcaphe

As far as I know, we can just use gtk_application_inhibit () and gtk_application_uninhibit (), however I'm not sure that it supports as many DEs as I did try to support with libcaphe.

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