Credentials autocompleting (secure version)

[CarloRamponi]

See #109

I've successfully integrated https://github.com/frankosterfeld/qtkeychain/ as @yuezk suggested.

Added two methods in the settings namespace:

bool secureSave(const QString &key, const QString &value);
bool secureGet(const QString &key, QString &value);

Credentials are saved when the login button is clicked in the StandardLoginWindow, and autocompleted when the same window is created. Maybe there is a better way to manage this, I'll leave it to the ones that have a deeper knowledge of the project (Perhaps we could try to log in without showing that window if the credentials are present in the wallet).

Disclaimer: only tested on Manjaro KDE

[yuezk]

Thanks for your PR, will process it this weekend.

[lazanet]

Hey @CarloRamponi, it seems that your PR Build is failing due to missing qttools5-dev package. The fix should be simple - adding that package here:

https://github.com/yuezk/GlobalProtect-openconnect/blob/fab25848e1c17ab8fb5c0806b65ceebf9cf52515/scripts/install-ubuntu.sh#L4-L9

[yuezk]

Thanks for your contribution!