Split microwaves into their own device type instead of the oven registry

Microwaves (combi and plain) were routed onto the oven registry (issue
#121), which meant entities carried oven-flavored keys (oven_state,
oven_mode, oven_setpoint) and inherited oven-specific behavior that's
wrong for this family: a 30-270C setpoint range instead of this family's
actual 40-200C, a cooking-mode list missing MicroWave/MicroWaveGrill/
MicroWaveConvection/KeepWarm entirely, and a lamp switch that read/wrote
the oven's 'UpperLamp' option token instead of this family's 'Lamp' token.

Adds a microwave device type (by_type/microwave.py,
capabilities/microwave.py) that reuses the oven board family's shared
operational-state/door/connected/recipe-cook capabilities but defines its
own cooking-mode, setpoint, and cavity capabilities with the corrected
bounds/vocabulary, plus a new power_level sensor for the cavity's Watt
setting that was previously unexposed.

README.md

@@ -25,7 +25,8 @@ Your state stays on your LAN: HA talks to the appliance over a direct DTLS sessi
 | Air purifier | `by_type/air_purifier.py` |
 | Dehumidifier | `by_type/dehumidifier.py` |
 | Dryer | `by_type/dryer.py` |
-| Oven (including combi microwaves) | `by_type/oven.py` |
+| Oven | `by_type/oven.py` |
+| Microwave (combi and plain) | `by_type/microwave.py` |
 | Gas cooktop (read-only burner status) | `by_type/cooktop.py` |
 | Range hood | `by_type/range_hood.py` |
 | Range | `by_type/range.py` |