Add support for using standard button IDs from automation

Some example uses:

	-- ~special snowflake~ OK/Cancel
	aegisub.dialog.display(config, {ok='Accept', cancel='Cancel'})

	-- On OS X the 'Help' button will be just a left-aligned ?
	aegisub.dialog.display(config, {ok='OK', cancel='Cancel', help='Help'})

	-- Each button in its own subtable to preserve passed order
	-- Unnecessary when using only IDed buttons since the passed order will
	-- be ignored in favor of the platform-standard order
	aegisub.dialog.display(config,
	  {{ok='Accept'}, {cancel='Cancel'}, {help='Help'}, 'Another Button'})

In some cases the passed labels will be ignored in favor of the
platform-standard labels.

Available IDs:
	ok
	yes
	save
	apply
	close
	no
	cancel
	help
	context_help

Note that many combinations of button IDs do not make sense and may have
strange effects.

Buttons with an ID of 'cancel' return false, as if ESC was pressed. A
button with an ID of 'close' results in that button being triggered on
ESC rather than cancel.

Buttons with an ID of 'ok', 'yes' and 'save' are set as the default
affirmative button for the dialog.

Closes #1609.
Don't invert horizontal scroll on the audio display on OS X

For some reason horizontal scroll comes pre-flipped.

Closes #1612.
non-pch compilation