Select target annotation when using freehand correction tools

[elodieburtin]

It is sometimes hard to obtain the desired result when using the freehand correction tools in an area containing several annotations: indeed, currently, it is not possible for the user to select which annotation should be corrected, and if the drawn polygon intersects with several annotations, all of them may be affected by the correction (which may result for example in undesired merge). It would be nice to have the possibility to select an annotation prior to the correction. Then, only the selected annotation would be affected by the freehand correction tools. If this improvement is implemented, the correction tool will no longer allow to merge annotations. Therefore, a new "Merge" tool should be made available.

[elodieburtin]

Possibility to select the annotation to correct implemented in 5c55bb0c2e40b83cc283d3c1a989b2f7f3772567. Merge tool requires further specifications (how to merge description, props, terms,..)