selection boxes shall auto-focus on selected item

[westram]

• when displaying a selection box with a selected item, the selected item often is not visible
• ARB does some auto-scrolling, e.g. when calling merge tool via
  arb_ntree ~/data/small.arb ../demo.arb
  but it fails to show the selected databases

Solution:

• write a function to query, whether some item is visible
• write a function for selection boxes to center any item (if it is not visible)
• Call the center function somewhere after startup and after any AWAR change

Additional things:

• If multiple items share the same value (regarding the value written to the AWAR): If you select the second item, the selection box jumps to the first item. Fix?
• Enable scrolling with wheel