In previous versions of ESS, there was a bug and ess-switch-process
wasn't supposed to pop-up the new buffer.
This has been fixed in a newer release of ess.
Now we start an R process with ess-request-a-buffer, with no-switch to nil
to pop-up the buffer, and 'ask-if-1 to always ask for a new process

No search is now search = NULL instead of search = F

Both get a red background, debugger is a top-side window
Set keybindings

- run undo-tree-visualize directly with SPC-u, instead of restarting undo-tree
- disable undo-tree-mode in in poly-markdown+r-mode-hook

Use persp-init-new-frame-behaviour-override instead of persp-init-frame-behavior

This reverts commit 24189875cf.

Introduces a bug :
- switching to a project goes to projects.org
- opening a file from the OS goes to another opened buffer

- set initial-buffer-choice to projects.org
- disable persp-mode opening of previous persp

This was due to C-/ ('undo) being remapped to text-scale-adjust,
thus triggering checks by undo-tree to not enable undo-tree in the buffer
Delete redundant binding for undo-tree-redo in org-mode-map
<SPC>u directly starts undo-tree-visualize instead of restarting the mode
markdown-R doesn't need to trigger undo-tree-mode anymore either