Security: Don't copy passwords to remote clients

When copying a password to the clipboard, password managers can set the
additional mime type "x-kde-passwordManagerHint" to tell klipper not to insert
secrets into its history.

This change adds an option to VNC connections to prevent krdc from copying
passwords to remote clients by checking the mime type before sending clipboard
contents.

Test:
Open a krdc connection to a remote client. Check the checkbox "Don't copy
passwords from supported password managers to remote" in the VNC connection setup
dialog.

On the local machine, run the password manager keepassxc, which is known to set
"x-kde-passwordManagerHint".

Select an entry from your password list and copy the password into the clipboard
by pressing Ctrl-C. Locally, the password will stay in clipboard for about 10
seconds but will not be inserted into klipper's history. Check the clipboard
on the remote machine. With the option unchecked the password will be there and in
klipper's history, with the option checked not.