Kwallet

Support of KWallet (KDEWallet/KF5Wallet) for Remmina

To enable this plugin during the compilation phase, you must have libkf5wallet-dev (deb/ubuntu) installed and run cmake with -DWITH_KF5WALLET=on

This MR also breaks the secret plugins API in order to split plugin loading and initialization into two different phases. A new secret plugin initialization priority parameter is introduced.

EDIT: I've updated the code so that -DWITH_KF5WALLET is ON by default if the KF5WALLET module is found