Inset Plots

This MR implements the feature "inset plot" allowing to add a plot area as a child of another plot area.

There two modes to create such an inset plot from the context menu of the plot area:

1. add inset plot - creates an empty default plot area inheriting the type (four axes, etc.) and the theme of the parent plot area
2. add inset plot with data - creates a copy of the plot area and add it as a child to the current plot area, the child inherits all properties of the parent plot area including the visualization of the data

It's also possible to copy&paste existing plot areas to create such an inset plot.

Example 1:

Example 2