Backend: add backend for libpurple

Add a new backend for folks. This collects information via the libpurple dbus server.

CC: @andrea.schaefer

backends/purple/meson.build

+purple_backend_name = 'purple'
+
+# NOTE: we don't export a backend library here.
+
+purple_backend_sources = [
+  'purple-backend-factory.vala',
+  'purple-backend.vala',
+  'purple-persona-store.vala',
+  'purple-persona.vala',
+]
+
+purple_backend_deps = [
+  backend_deps,
+]
+
+purple_backend_vala_flags = [
+]
+
+purple_backend_c_flags = [
+  '-include', 'config.h',
+  '-DBACKEND_NAME="@0@"'.format(purple_backend_name),
+  '-DG_LOG_DOMAIN="@0@"'.format(purple_backend_name),
+]
+
+purple_backend = shared_library(purple_backend_name,
+  purple_backend_sources,
+  dependencies: purple_backend_deps,
+  vala_args: purple_backend_vala_flags,
+  c_args: purple_backend_c_flags,
+  name_prefix: '',
+  install_dir: folks_backend_dir / purple_backend_name,
+  install: true,
+)
+
+purple_backend_dep = declare_dependency(
+  link_with: purple_backend,
+  include_directories: include_directories('.'),
+)
+
+folks_backends += purple_backend