[roll third_party/pigweed] pw_console: PluginMixin and plugin infrastructure

- PluginMixin class for handling plugin background tasks and
  thread startup.

- plugins.rst containing the beginnings of how to author plugins.
  As example plugins are added this doc will be fleshed out more.

- Update PwConsoleEmbed with functions to add plugins:
  - add_window_plugins
  - add_top_toolbar
  - add_bottom_toolbar

- style.py: Add named color classes so plugins can easily make use
  of the current theme colors.

- checkbox.py: Make key legends on buttons optional for plugin
  authors.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/68681

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 3b4f89a26c4d16f..0a35c93e821fec5
Roller-URL: https://ci.chromium.org/b/8828373393050482593
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8828373393050482593
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3ae38d02a321404c69767deecf19f1a2ebfcb165
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/72866
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 3b4f89a..0a35c93 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 3b4f89a26c4d16f7dd5cb0b13d2a932e8be9dfde
+Subproject commit 0a35c93e821fec501901301d6aa6c6ea3e64ba8d