GObject-Introspection fixes

• d/control: Use ${gir:Depends} in -dev package instead of hard-coding

  This avoids mistakes, and will make it easier to implement the <!nogir> build-profile at a later date, if desired.

• d/control: Add ${gir:Provides} to -dev package

  This ensures that it will provide the appropriate systematic name derived from its .gir file, and signals that it was not built with GObject-Introspection disabled (which is not currently set up, but could be added later via the nogir build-profile).

• d/control: Explicitly build-depend on gir1.2-*-dev packages

  These virtual package names can be mechanically derived from the uses of --include=GObject-2.0 and --include=Gtk-3.0 in the build system, and are the preferred way to ensure that the required GIR XML is present at build-time.

• d/control: Drop obsolete B-D on libgirepository1.0-dev

  This transitional package can't be multiarch-friendly for historical reasons. In this package, its only value was to pull in the code generation tools and GLib's own GIR XML, but those are now depended on via gobject-introspection and gir1.2-gobject-2.0-dev respectively.

  Closes: #1118854

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I've confirmed with diffoscope that this results in the same binaries as rebuilding the current source, with only the following metadata changes:

• libayatana-appindicator3-dev Depends on gir1.2-gobject-2.0-dev, gir1.2-gtk-3.0-dev and dependencies are re-ordered
• libayatana-appindicator3-dev Provides gir1.2-ayatanaappindicator3-0.1-dev (= 0.5.94-1)
• buildinfo: gir1.2-girepository-2.0, gir1.2-girepository-2.0-dev, libgirepository-1.0-dev, libgirepository1.0-dev no longer installed (#1118854)

in addition to these metadata changes from c31fa839:

• gir1.2-ayatanaappindicator3-0.1 is Multi-Arch: same (#1072967)
• libayatana-appindicator-doc is Multi-Arch: foreign (ditto)
• libayatana-appindicator3-1 and its debug symbols are Multi-Arch: same (ditto)
• libayatana-appindicator3-dev is Multi-Arch: same (ditto)