Speed up Lua scripts startup

When images are imported from the command line at darktable startup Lua doesn't initialize quick enough to capture import events so on a multi image import the first 5 - 10 images may not be recognized.

The culprit is script_manager and the fact that it doesn't (re)start running scripts until the GUI initializes.

A possible work around that has been tested is adding a post-import-image event "buffer" into data/luarc to capture an import from the command line and make the image list available to scripts starting up and then clear the"buffer" after script_manager is fully initialized.

The best fix is to rework the way that script_manager starts scripts.

darktable の起動中に複数画像のコマンドラインインポートを再現し、data/luarc と併せて script_manager を調査する。起動時のスクリプト初期化と post-import-image イベントがどのように相互作用するかを確認する。GUI の初期化回避策に頼らず、最初にインポートされた画像をスクリプトから利用できれば完了とする。