Add the "set style source" command

Add the "set style source" command

This adds "set style source" (and "show style source") commands.  This
gives the user control over whether source code is highlighted.

gdb/ChangeLog
2019-03-14  Tom Tromey  <tromey@adacore.com>

	* NEWS: Add item for "style sources" commands.
	* source-cache.c (source_cache::get_source_lines): Check
	source_styling.
	* cli/cli-style.c (source_styling): New global.
	(_initialize_cli_style): Add "style sources" commands.
	(show_style_sources): New function.
	* cli/cli-style.h (source_styling): Declare.

gdb/doc/ChangeLog
2019-03-14  Tom Tromey  <tromey@adacore.com>

	* gdb.texinfo (Output Styling): Document "set style source" and
	"show style source".

gdb/testsuite/ChangeLog
2019-03-14  Tom Tromey  <tromey@adacore.com>

	* gdb.base/style.exp: Add "set style sources" test.