The big plugin refactor

Since we're about to add a second kind of plugin for "policy engines" (which implement call blocking; see !588 ) the plugin infrastructure gets a major overhaul.

• A CallsPluginManager is introduced which deals with all plugins and keeps a GListModel of providers (now) and policy engines (with !588).
• Use GtkFlattenListModel in CallsManager for all our origins (calls_plugin_manager_get_providers() and individual calls_provider_get_origins ()
• Use GtkFilterListModel in CallsManager for per-protocol (tel, sip, sips) list models
• Use GtkFilterListModel in CallsAccountOverview for CallsAccountProvider and CallsAccounts
• Adjust to changes everywhere
• Make plugin desktop files (mm.plugin, sip.plugin, etc) translatable so we can show their name and description in the UI

TODOs as a follow-up

• Rebase !588 on top of this
• Make plugins derive CallExtension (instead of CallsProvider):
  • instead of e.g. CallsProvider, add API to get provider with calls_extension_get_provider()
  • Add calls_extension_get_account_widget () to get the account widget instead of having to go through the CallsProvider
  • Add calls_extension_get_provider_preferences_widget () (to be embedded in the preferences) and implement it for (part of) our providers (where it makes sense, because there's configurable things)
  • Add calls_extension_get_modem_preferences_widget() to show some info and let the user configure call waiting, voice mail, checking balance, etc?
  • This also allows to get rid of the #ifdef FOR_TESTING in provider implementations and revert d3a93bd6 for improved (reported) code coverage \o/
• Add preference window with the following panes (ish) (now in !690 )
  • General settings like Enable/disable plugins)
  • Call blocking (see #370)
  • Modems (e.g. setup call waiting, voicemail, ..)
  • Accounts (CallsAccountOverview moves there)