option to search data outside the current selection when performing a find

Jalview's find dialog currently limits any searches to the active selection, if one exists, although this is not made clear when the user hits the 'Find'/'Find All' buttons. This is useful but not perfect - sometimes one may wish to find things in order to add them to the current selection (particularly if JAL-345 is implemented), or preserve the selection for other purposes.

Two refinements seem possible:
1. if the user's search reaches the end of the active selection, they are given the option to repeat the search on the rest of the alignment.
2. A checkbox is provided to search the alignment rather than the current selection.