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