Clicking on labels to collapse/expand groups

[Istador]

This commit fixes #51

[stefanocudini]

good idea

[Istador]

Ah, good to see that you're back. I assumed the project to be dead after about four months of inactivity and forked it.

---

You might want to integrate Istador@5477189 (don't merge it directly, because I changed other things too). It adds a new optional option groupCheckboxes, that when set to true adds indeterminate checkboxes to the group label to select/deselect all layers in a group at once. A demonstration of it can be seen here: https://pine.blackpinguin.de/

I took that feature from the other fork https://github.com/syonfox/leaflet-panel-layers, that also changed a few other things that were incompatible with my project (e.g. moving the checkboxes to the left where they collide visually with the layer icons).

---

I also updated the dependencies to newer versions of grunt. See the changes to the Gruntfile.js and the package.json at Istador@9138d27 (one of the dependencies, uglify or cssmin, doesn't support the banner option anymore, so I changed it to use grunt-banner instead).

[stefanocudini]

hi @Istador
I have had serious health problems unfortunately :-/
now I will go back to reading the issues