Update KWalletmanager docbook to 16.04

remove header entities already in kdoctools
bump date + releaseinfo
new screenshots
add info about default wallet kdewallet
add encrypted import + export, remove obsolete "Save as"
correct location of .kwl file
remove systen tray icons, not shown by default any longer
remove commented section about ksecrets
remove dead link to techbase
REVIEW:128378