pw_console: LogContainer, rendering and scrolling

New LogContainer class which:
- Holds log lines for the parent LogPane
- Tracks the currently selected line
- Handles scrolling
- Renders the number of log lines that will fit in the LogPane window

No-Docs-Update-Reason: Log viewer functionality still WIP.
Change-Id: Iba5c290daad44a49650cc537417e53e99d83a29f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49024
Pigweed-Auto-Submit: Anthony DiGirolamo <tonymd@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Joe Ethier <jethier@google.com>
diff --git a/pw_console/py/BUILD.gn b/pw_console/py/BUILD.gn
index 10865c2..b53b2a6 100644
--- a/pw_console/py/BUILD.gn
+++ b/pw_console/py/BUILD.gn
@@ -25,6 +25,7 @@
     "pw_console/help_window.py",
     "pw_console/helpers.py",
     "pw_console/key_bindings.py",
+    "pw_console/log_container.py",
     "pw_console/log_pane.py",
     "pw_console/pw_ptpython_repl.py",
     "pw_console/repl_pane.py",