Fix "list" command in the TUI

Fix "list" command in the TUI

PR tui/18932 notes that "list" no longer works in the TUI.  At some
point in the past, it switched the TUI source window to show the
specified source; but now this source briefly flashes before the TUI
reverts to showing the current stack frame's source.

This patch fixes this bug by introducing a new observer that notices
when the user selected context has changed.  Then, the existing
before-prompt observer is updated to request the correct update:
either one based on the current stack frame, or one based on the
user's source symtab_and_line.

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

	PR tui/18932:
	* tui/tui-hooks.c (tui_refresh_frame_and_register_information):
	Rename parameters.  Handle the not-from-stack-frame case.
	(from_stack, from_source_symtab): New globals.
	(tui_before_prompt, tui_normal_stop): Update.
	(tui_context_changed, tui_symtab_changed): New functions.
	(tui_attach_detach_observers): Attach new observers.

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

	* gdb.tui/list-before.exp: New file.

Change-Id: I62013825f6c1afdd568a1c7a8c019b0c881131af