Webtiles and console both have a monster list at the bottom-right of
the screen which lists all monsters in view color-coded by rough
threat level. Many players have expressed a desire for something
similar in offline tiles, to provide always-visible indicators of the
threat levels of all enemies in view. This commit adds translucent
colored backgrounds -- dark grey, light grey, yellow, and red --
to indicate trivial, easy, tough, and nasty monsters respectively.

Additionally, these indicators are configurable with a new option in
the options file, "tile_show_threat_levels". Each type of indicator
can be independently enabled or disabled by including or omitting it
in the option file line. For example, the following...

"tile_show_threat_levels = tough nasty"

...will show only yellow (tough) and red (nasty) threat indicators.

I have experimented with various styles of indicators, as shown in the
following thread:

https://crawl.develz.org/tavern/viewtopic.php?f=8&t=26439

I am open to changing back to a different style, or even making
multiple styles available and letting the player choose which style to
use, if any. This style is the best in my opinion, as it is very
noticeable while being subtle enough not to interfere with any other
monster or monster-associated graphics. I have playtested this feature
through a 6-rune run with no problems, technical nor cosmetic.