Size of resizable TopBottomPanel is not persisted since 0.19 any more.

Until version 0.18, the size of resizable TopBottomPanel was persisted. Since 0.19, it is not persisted anymore.

**To Reproduce**
Steps to reproduce the behavior:

1. git clone https://github.com/ruimo/egui-resize-sample.git
2. cd egui-resize-sample
3. cargo run
4. Resize top/bottom pane.
5. Close sample app and re-run.
6. The sizes changed at step 4 are retained.
7. Edit Cargo.toml and update the dependencies section to 'eframe = { git = "https://github.com/emilk/egui", branch = "master" }'.
8. cargo update
9. cargo run
10. Repeat step 4-6. The sizes are not retained any more.

**Expected behavior**
Step 4-6 should behave as same as the v0.18.

**Desktop (please complete the following information):**
- OS: Linux dev 5.15.0-46-generic #49-Ubuntu SMP Thu Aug 4 18:03:25 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
- Version 0.19