Updates theme names to be compatible with Godot 3.5 (no spaces),
consistent with other Godot theme item names (snake_case), and
match the color names listed on the
[ANSI escape code wikipedia page](https://en.wikipedia.org/wiki/ANSI_escape_code#3-bit_and_4-bit).
Deprecates the old names and warns users to change them.
- Don't add the bell to the archive to keep it small a simplify
licensing. Also bells seem to be rarely used with terminal emulators.
- Don't play the bell directly from the Terminal node by adding an
AudioStreamPlayer, but make it easy to tune the "bell" signal behavior
from the Terminal node so that only an AudioStreamPlayer node's play()
method needs to be connected to it.
- Keep the bell.wav sound around for testing/demo.
Keeps the archive small and licensing simpler.
Keep around the fancy fonts such as Noto Color Emoji and Unifont as they
are useful for testing.
With the xrdb import plugin making it easy to import themes, there is
little point in keeping other pre-defined themes around, so remove
these.
