Bitcoin-Qt: fix known addressbook bugs

- add qSort() for cachedAddressTable, as qLowerBound() and qUpperBound()
  require the list to be in ascending order (see
  http://harmattan-dev.nokia.com/docs/library/html/qt4/qtalgorithms.html#qLowerBound)
- add a new check in AddressTableModel::setData() to just return, when no
  changes were made to a label or an address (prevents entry duplication
  issue)
- remove "rec->label = value.toString();" from
  AddressTableModel::setData() as the label gets updated by
  AddressTablePriv::updateEntry() anyway (seems @sipa added this line via
  https://github.com/bitcoin/bitcoin/commit/1025440184ef100a22d07c7bb543ee45cf169d64#L6R225)
- add another new check in AddressTableModel::setData() to just return, if
  a duplicate address was found (prevents address overwrite)
- add a new check to EditAddressDialog::setModel() to prevent setting an
  invalid model
- re-work the switch-case statement in AddressTableModel::accept() to
  always break (as return get's called anyway) and order the list to match
  the enum definition
- make accept() in editaddressdialog.h a public slot, which it should be
- misc small coding style changes

Intended to fix: #2137 and #1839.