gdbsupport: introduce struct observer

gdbsupport: introduce struct observer

Instead of using a pair.  This allows keeping more data per observer in
a structured way, and using field names is clearer than first/second.

gdbsupport/ChangeLog:

	* observable.h (class observable) <struct observer>: New.
	<detach, notify>: Update.
	<m_observers>: Change type to vector of observers.

Change-Id: Iadf7d1fa25049cfb089e6b1b429ddebc548825ab