Allow editing a title clip from the timeline with double click

We already have the action to edit a clip that works from the timeline if we define a shortcut for it:

addAction(QStringLiteral("edit_clip"), i18n("Edit Clip"), QIcon::fromTheme(QStringLiteral("document-open")));

Allow this to be called on double click on the title clip in the timeline.

Should this be renamed to Edit Title Clip, or will this work with other kind of clips in the future (color, nested, etc)?