ConnMan: use separate text entries for each proxy server type

As ConnMan allows us to store any arbitrary string value as a proxy server
URL we need to take steps in the UI to encourage the user to enter
appropriate values. This change adds separate entries for each proxy
protocol, spin buttons for port numbers and uses placeholder text to
explain appropriate values.
ConnMan: fix detection of changes in entries

Using the notify signal of the text-length property isn't very reliable,
i.e. if the entire contents of the entry are selected and deleted the signal
is not emitted. Instead use the 'changed' signal of the implemented
GtkEditable interface.
Merge pull request #4 from incandescant/3.8.3-devel

Proxy configuration UI changes