Fix SVG background rect placement, improve test coverage

- Background rects now rendered before text elements (valid SVG)
- Add TwoWayDict tests for reassign and duplicate value cases
- Test coverage at 80%

Bump version to 0.3.2

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.1"
+version = "0.3.2"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"