New option: auto-generate layout names

[bmag]

If spacemacs-auto-generate-layout-names is non-nil, and the user tries to open a layout in a position that doesn't yet have a layout (e.g. with SPC l 2), then create a new layout with an automatically generated name. I tried to choose names that are familiar and easy to remember. To enable the feature, add the following to user-config:

(setq spacemacs-auto-generate-layout-names t)

Known issues:

• persp-mode doesn't really have a notion of a numbered layout. Because of this, creating a layout with SPC l 7 (7 is only an example) will give it a name suitable for a 7th layout, but the actual position of the layout may be smaller. For example, if the user only had 4 layouts, then the "7th" layout is created in position 5.
• the new layout is created with a single window, but it doesn't show the home buffer. Instead, it will show the buffer of the previous layout's selected window. This is a difference I do not understand (or researched) between (persp-switch "some-new-name") and (persp-switch nil).

[JAremko]

Nice addition. Thanks!

[TheBB]

Do you mean grab bag? :-)

[syl20bnr]

spacemacs-auto-generate-layout-names could be a dotspacemacs variable, check for the shape of other layout related dsm variables.

[bmag]

@TheBB lol, of course
@syl20bnr ok, updated accordingly. Original version was modelled after spacemacs-spaceline-additional-segments.

[TheBB]

Thanks! Cherry-picked in develop. You can safely delete your branch.

I had to apply a fix: d4b0044

Bizarrely, by default, persp-get-by-name will not return nil if it can't find a persp, but :nil.