Fix type of signal IDs, make disconnection more obviously correct

[smcv]

• Fix type used for signal IDs

  While trying to investigate intermittent segfault during automated tests #169 I noticed that inputcapture was using
  guint instead of the correct gulong for GObject signal IDs, which could
  conceivably result in a signal ID being truncated and the wrong signal
  being disconnected (although in practice signal IDs are allocated
  starting from 0, so this is unlikely). The same pattern is seen in all
  of the other portals.

  Using the correct type also allows g_clear_signal_handler() to be used,
  for idempotent signal disconnections.

• Consistently use g_clear_signal_handler to disconnect GObject signals

  This is more concise than writing out equivalent code every time,
  and ensures that signal disconnection is idempotent and that the correct
  type (gulong) is always used for the signal ID.