Move file execute region back to C, fixing bugs

We moved this to Python too quickly, causing the following regressions:
* No auto completion for file names
* Additional handling not applied on changes, like automatic extension
  appending (see file_filename_enter_handle)
* Red highlight missing when the file name already exists

Note that earlier (before the file browser redesign), this didn't use
the panel and layout code at all. So even if it's still not in Python,
at least it's integrated into regular panel management now.
OS-specific ordering of the open and cancel button is kept.

Fixes T69457.

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