Support renaming layers directly in the Manage layers list

[waldyrious]

Is your feature request related to a problem? Please describe.
Currently to rename a layer I need to open its editing form and change the title. This is a bit cumbersome and requires more steps than would be ideal.

Describe the solution you'd like
In many layered editing programs (e.g. GIMP, Inkscape) it's possible to simply double-click the layer name in the list of layers and the text changes to edit mode, so that one can edit the title inline. I'd like uMap to also have this functionality.

Describe alternatives you've considered
If it's easier to implement, a Rename icon next to the layer name, that when clicked shows a pop up to edit new layer name, would also be more convenient than the current workflow. But inline editing would of course be preferable.