Fix #1395 "Controls form: no alerts for XBL components"

Regression due to 898aa8f088ec2f52effbb2ad8d473598c96399aa.
Fix #1394 "Wizard: click on error in error summary does not switch section"

- make fr:number and fr:currency focusable and remove custom focus code
- these 2 controls use a hidden xf:input control and a visible HTML input control
- generic focus code in xforms.js takes care of focusing and first visible descendant
- other XBL controls don't have this issue because they use visible nested XForms controls