Closes: #5 and 6

Improving support for handling certificate selection. If a user selects a pkcs12 certificate envelope, the nma-file-cert-chooser implementation will gray out the key label/button while letting the user type in the password. If the certificate changes, it will clear the key input; if the key changes, it will clear the password input.

The heavy lifting of this logic was moved to the nma-cert-chooser implementation instead of imposing undue burden on the network-manager-openvpn plugin (and others which may want to use this, like network-manager-sstp plugin).

Tested this with an unmodified version of network-manager-openvpn plugin on ubuntu to ensure proper operation before and after the change.