Components.WebView: Remove selection_changed signal

Now that vala only notifies on property changes when the value actually changes, and GLib emits a warning when connecting to notify signals for properties that don't exist, it's safer to use notify signals for property changes, and hence this isn't needed any more.

This patch removes the signal, ensures the has_selection gets set correctly, and updates signal listeners to use notify instead.

Fixes #1058 (closed), #1118 (closed) and possibly others caused by regression in commit fb96676f