Preserve custom Locate Current Document action's shortcut

This shortcut can be configured in the Configure Keyboard Shortcuts
window. The custom-set shortcut works at first. But once KDevelop is
restarted, the shortcut is reset to the default Ctrl+<.

Calling KActionCollection::setDefaultShortcut() instead of
QAction::setShortcut() preserves the shortcut between KDevelop launches.