Add certificate verification for RDP connections

This merge request adds dialogs for verification of certificate of remote RDP connection. It also adds dialog for warning about changed certificate of such connection. It tries to implement the design by Jimmac from #119 (closed). It still uses gtk3 so it does not look exactly the same as the design though.

E.g. the icon in the certificate changed verification dialog is not the same as the on on the mock-up.

It still misses reworked authentication dialog but I plan to implement the redesign in another merge request and place it into the same dialog as the verification dialogs, just another page in the GtkStack.

Since this merge request depends on changes in gtk-frdp, it can be merged only after the changes are merged into gtk-frdp and Connections needs to be updated with the version of gtk-frdp.

This merge request fixes #119 (closed).

Here are some screenshots: