Using `g_clear_handle_id()` in combination with `g_source_remove()` can
save a lot of boiler plate code.
This removes about 110 lines of code for free.
Since the entry already shows an apply button, update hostname only
after the apply button is clicked. It's not required to update
hostname after every character change.
This is only the bare minimum to make everything under panels/common
build. Since these widgets are used by the main window, port them
first.
CcTimeEntry was particularly hard to port. That's because GtkEntry
is a final class now. Overall, though, I'm happy with how it turned
out to be - much cleaner, less code, more obvious.
