do not add child node if node is collapsed

[AngeloR]

you can only add child nodes if the node is expanded and you can see its children. Otherwise the cursor just disappears and you have no idea what's happening.

[github-actions]

Build for macos-latest is done.
Uploaded Assets:
["/Users/runner/work/outliner/outliner/src-tauri/target/aarch64-apple-darwin/release/bundle/dmg/Outliner_0.0.2_aarch64.dmg","/Users/runner/work/outliner/outliner/src-tauri/target/aarch64-apple-darwin/release/bundle/macos/Outliner.app"]

[github-actions]

Build for ubuntu-22.04 is done.
Uploaded Assets:
["/home/runner/work/outliner/outliner/src-tauri/target/release/bundle/deb/Outliner_0.0.2_amd64.deb","/home/runner/work/outliner/outliner/src-tauri/target/release/bundle/rpm/Outliner-0.0.2-1.x86_64.rpm","/home/runner/work/outliner/outliner/src-tauri/target/release/bundle/appimage/Outliner_0.0.2_amd64.AppImage"]

[github-actions]

Build for windows-latest is done.
Uploaded Assets:
["D:\a\outliner\outliner\src-tauri\target\release\bundle\msi\Outliner_0.0.2_x64_en-US.msi","D:\a\outliner\outliner\src-tauri\target\release\bundle\nsis\Outliner_0.0.2_x64-setup.exe"]