PR: Support DatetimeIndex in Variable Explorer

[jitseniesen]

Support variables of panda's DatetimeIndex type in variable editor, and use the dataframe editor to display them. Fixes #3380.

[jitseniesen]

I actually wrote this a while ago, while we were in freeze for the 3.0 release.

There are a number of places that have to be changed to support a new type in the variable explorer. There are also a number of issues which ask to support more types: #3758, #3562, #2824, #2510, #2355, etc. Can we make it easier to add support for a new type, possibly in a plugin?

[ccordoba12]

Can we make it easier to add support for a new type, possibly in a plugin?

That's my plan for Spyder 4.0 :-) I think we could add a function called register_object to spyder/api/variableexplorer.py that would take care of doing all you've done here.

But for that we need to restructure how we dispatch objects to viewers in the Variable Explorer code, so it can be easily extended.

[ccordoba12]

@jitseniesen, this has conflicts now.

I'm leaving this for Spyder 4.0 because your PR is against master. If you want this for Spyder 3, please redo it against 3.x :-)

[jitseniesen]

PR redone against 3.x and conflicts resolved.

[ccordoba12]

Thanks a lot! Just in time for 3.1 :-)