Avoid mixing automatic assigned IP addresses with static IP addresses

[agners]

Currently the Network settings dialog applies the current IP settings to the static address fields. This is somewhat problematic when DHCP has been used before: When switching from DHCP -> Static, the suggested IP address/Netmask is from the DHCP pool. When applying this IP address, it can cause IP address conflicts in the future.

However, it is convenient to have parts of the network address already present. Ideally we should just clear out the host part. Also we should not clear the IP address in case the setting was static at the beginning (e.g. Static -> DHCP -> Static should not lead to a cleaned IP address).

Ideally, the logic should be as follows: If the setting which the backend returned was "DHCP" and new setting is "Static" then filter the IP address according to netmask (e.g. 192.168.1.123/24 -> 192.168.1.x/24).

This makes it less likely that people accidentally/unknowingly use an IP in the DHCP pool.