Update love.plot to be compatible with upcoming ggplot2 version

[teunbrand]

Hello there,

We have been preparing a new release of ggplot2 and during a reverse dependency check, it became apparent that the prospective ggplot2 3.5.0 would break cobalt.

In the next version, a plot can have legends at multiple locations. This goes against the expectation in love.plot() that there is one, and only one, legend per plot. This PR updates love.plot() to deal with the new legend structure in ggplot2.

To test the code changes with the release candidate, you can install it with the code below:

remotes::install_github("tidyverse/ggplot2", ref = remotes::github_pull("5592"))

The release of ggplot2 3.5.0 is scheduled for the 12th of Februari. The progress of the release can be tracked in tidyverse/ggplot2#5588. We hope that this PR might help cobalt get out a fix if necessary.

[ngreifer]

Thank you very much for this!