[shotwell] plugins: Add override path

[Jens Georg <jensgeorg src gnome org>]

commit b9bd1a9d26e6d80f7d20764607d87c27a6d63917
Author: Jens Georg <mail jensge org>
Date:   Fri Jul 6 11:59:07 2018 +0200

    plugins: Add override path
    
    Add possibility to add another search path for plugins

 src/plugins/Plugins.vala | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/src/plugins/Plugins.vala b/src/plugins/Plugins.vala
index 5f6613cf..6aff4616 100644
--- a/src/plugins/Plugins.vala
+++ b/src/plugins/Plugins.vala
@@ -112,6 +112,10 @@ private Gee.HashSet<string> core_ids;
 
 public void init() throws Error {
     search_dirs = new File[0];
+    unowned string plugin_dir = Environment.get_variable("SHOTWELL_PLUGIN_PATH");
+    if (plugin_dir != null && plugin_dir != "") {
+        search_dirs += File.new_for_commandline_arg(plugin_dir);
+    }
     search_dirs += AppDirs.get_user_plugins_dir();
     search_dirs += AppDirs.get_system_plugins_dir();