Plugin: Add new Composer object and Application.new_composer method

This allows plugins to create and show new composers for a specific
account.

po/POTFILES.in

@@ -94,6 +94,7 @@ src/client/folder-list/folder-list-tree.vala
 src/client/plugin/plugin-account.vala
 src/client/plugin/plugin-application.vala
 src/client/plugin/plugin-button.vala
+src/client/plugin/plugin-composer.vala
 src/client/plugin/plugin-contact-store.vala
 src/client/plugin/plugin-email-extension.vala
 src/client/plugin/plugin-email-store.vala

src/client/application/application-plugin-manager.vala

@@ -64,6 +64,15 @@ public class Application.PluginManager : GLib.Object {
             this.action_group_name = plugin.get_module_name().replace(".", "_");
         }
 
+        public Plugin.Composer new_composer(Plugin.Account source)
+            throws Plugin.Error {
+            AccountContext? account = this.folders.get_account_context(source);
+            if (account == null) {
+                throw new Plugin.Error.NOT_SUPPORTED("No such account");
+            }
+            return new ComposerImpl(this.backing, account);
+        }
+
         public void register_action(GLib.Action action) {
             if (this.action_group == null) {
                 this.action_group = new GLib.SimpleActionGroup();
@@ -141,6 +150,30 @@ public class Application.PluginManager : GLib.Object {
     }
 
 
+    private class ComposerImpl : Geary.BaseObject, Plugin.Composer {
+
+
+        private Client application;
+        private AccountContext account;
+
+
+        public ComposerImpl(Client application, AccountContext account) {
+            this.application = application;
+            this.account = account;
+        }
+
+        public void show() {
+            var composer = new Composer.Widget(
+                this.application, this.account.account, NEW_MESSAGE
+            );
+            var main_window = this.application.get_active_main_window();
+            main_window.show_composer(composer, null);
+            composer.load.begin(null, false, null, null);
+        }
+
+    }
+
+
     /** Emitted when a plugin is successfully loaded and activated. */
     public signal void plugin_activated(Peas.PluginInfo info);
 

src/client/meson.build

@@ -113,6 +113,7 @@ geary_client_vala_sources = files(
   'plugin/plugin-account.vala',
   'plugin/plugin-application.vala',
   'plugin/plugin-button.vala',
+  'plugin/plugin-composer.vala',
   'plugin/plugin-contact-store.vala',
   'plugin/plugin-email-extension.vala',
   'plugin/plugin-email-store.vala',

src/client/plugin/plugin-application.vala

@@ -14,6 +14,15 @@
 public interface Plugin.Application : Geary.BaseObject {
 
 
+    /**
+     * Constructs a new, blank composer for the given account.
+     *
+     * The composer will be initialised to send an email from the
+     * given account. This may be changed by people before they send
+     * the email, however.
+     */
+    public abstract Composer new_composer(Account source) throws Error;
+
     /**
      * Registers a plugin action with the application.
      *

src/client/plugin/plugin-composer.vala

@@ -0,0 +1,24 @@
+/*
+ * Copyright © 2020 Michael Gratton <mike@vee.net>
+ *
+ * This software is licensed under the GNU Lesser General Public License
+ * (version 2.1 or later). See the COPYING file in this distribution.
+ */
+
+/**
+ * An object representing a composer for use by plugins.
+ */
+public interface Plugin.Composer : Geary.BaseObject {
+
+    /**
+     * Causes the composer to be made visible.
+     *
+     * The composer will be shown as either full-pane and in-window if
+     * not a reply to a currently displayed conversation, inline and
+     * in-window if a reply to an existing conversation being
+     * displayed, or detached if there is already an in-window
+     * composer being displayed.
+     */
+    public abstract void show();
+
+}