Add observable to watch current source symtab

This patch adds an observable, so that a later patch can change the
TUI to notice when the user has selected a new source symtab, say via
"list".

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* source.c (struct current_source_location) <set, symtab, line>:
	New methods.
	<m_symtab, m_line>: Rename.  Now private.
	(get_current_source_symtab_and_line)
	(set_default_source_symtab_and_line)
	(set_current_source_symtab_and_line)
	(clear_current_source_symtab_and_line, select_source_symtab)
	(info_source_command, print_source_lines_base)
	(info_line_command, search_command_helper): Update.
	* observable.h (current_source_symtab_and_line_changed): Declare
	observable.
	* observable.c (current_source_symtab_and_line_changed): Define
	observable.

Change-Id: I3c0f6b40f2df84b590bdf5b5ec5ccd8423bb7f22