gdb/tui: Link source and assembler scrolling .... again

gdb/tui: Link source and assembler scrolling .... again

Until recently when the source window was scrolled the assembler
window would scroll in sync - keeping the disassembly for the current
line in view.

This was broken in commit:

  commit b4b49dcbff6b437fa8b4e2fc0c3f27b457f11310
  Date:   Wed Nov 13 16:47:58 2019 -0700

      Don't call tui_show_source from tui_ui_out

This commit restores the synchronised scrolling and also maintains the
horizontal scroll within the source view when it is vertically
scrolled, something that was broken before.

This commit does not mean that scrolling the assembler view scrolls
the source view.  The connection this way never existed, though maybe
it should, but I'll leave adding this feature for a separate commit.

gdb/ChangeLog:

	* tui/tui-source.c (tui_source_window::do_scroll_vertical): Update
	all source windows, and maintain horizontal scroll status while
	doing so.

gdb/testsuite/ChangeLog:

	* gdb.tui/basic.exp: Add more scrolling tests.

Change-Id: I250114a3bc670040a6a759d41905776771b2f818