Label support

[sebasmonia]

Add support to list labels and get by label.

[sebasmonia]

Seems like yet another tabulated list is coming, this one should look pretty similar to the shelvesets one so there's potential to recycle that one with some flags/params to determine if it's labels/shelves mode.

Label details gets the entire list of files affected, which can become unwieldy.

Creating labels seems simple enough except for the list of items to label, whether is recursive or not, etc. Seems like it can get quite hairy.

Moving to v2 :)

[sebasmonia]

Consider a "starter version" that helps you build the parameters you pass to the tf process, bare bones but you don't have to leave Emacs at least.