Allow changing options of multiple skeletons at once

[misaki-eymard]

Currently, when multiple skeletons are selected, the options are hidden as shown in the following image, so it is not possible to check/uncheck the Export option or change the Reference scale at once:

This can be a hassle in projects with a very large number of skeletons, so it would be helpful if the options field remains visible when multiple skeletons are selected, like in the image below:

Related forum thread:
https://esotericsoftware.com/forum/d/26718-export-selection-and-reference-scale-for-multiple-skeletons-at-once