nav: jumping from the decompiler stays in the decompiler

Following a reference or picking an xref target from the pseudocode dropped you
back into the linear listing. Now such jumps stay in the decompiler when the
target is a decompilable function, landing on the pseudocode line that matches
the target address; non-decompilable targets still fall back to the listing.

* NavEntry gains a `view` field ("listing"/"decomp"); _open_entry restores an
  entry in whichever view it was seen in (so back/forward also keep pseudocode).
* _do_navigate/_goto_ea take prefer_decomp; when set and the target function
  decompiles, _open_decomp_entry opens it in the decompiler as a real nav-history
  push (snapshotting the source pseudocode position so 'back' returns to it).
* _follow_decomp and _on_xref_chosen pass prefer_decomp when the source view is
  the decompiler.

Verified: follow a ref from decomp -> lands in the target's pseudocode; 'back'
returns to the source pseudocode; listing follow still lands in the listing.
decomp_nav/decomp_follow_self/follow_xrefs/xref_labels/view_toggle/disasm_nav/
search/mouse/listing_view/func_banners/continuous_view 62/0.

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