Layout doesn't restore XY plots when multiple sources with prefixes are used

[lukegluke]

1. Add at least two files (no matter what, a use motor_data.csv and its copy)
2. Create XY plot with any data and also other simple data plot.
3. Save layout (with or without data source, it doesn't matter).
4. Delete everything and load the layout back.
5. XY plots will be empty while simple plots will have data.

Add prefixes ("motor_data1/") to curve_x and curve_y fields manually in layout file fix the issue, so it looks like PlotJuggler doesn't but should add prefixes for curves x and y on saving.

Everything is ok if using only single data file, including files like turtle.csv that has prefixed data inside, PlotJuggler adds these prefixes to curves in layout correctly.

Version 3.6.0, Windows 10