Avoid duplicate tray windows

[gaoflow]

Summary

• add a shared helper to reuse an already-open form of the requested type
• bring the existing paste dialog or settings wizard to the front instead of opening duplicates from the tray icon/menu

Fixes #100

Validation

• git diff --check

Not run locally: dotnet restore / Windows build, because this environment does not have dotnet, msbuild, xbuild, or csc installed.