core.shared: silence "missing preview script" message @ start

This bug caused an error message to appear on every start of ranger, if
the user has no scope.sh, because "use_preview_script" is set to true
but "preview_script" is None.

The expected behaviour would be to silently ignore it, to stop annoying
people who don't want scope.sh.
config/rc.conf: added comment to "preview_images" setting
config/rc.conf: update comment of "colorscheme" setting
config/rc.conf: typo
widgets.statusbar: remove superfluous space

When display_size_in_status_bar is turned off, there was an extra space
where the size would usually be.
config/rc.conf: update comment of "shorten_title" setting
widgets.browserview: fix border drawing in all corner cases

Border drawing depends on many things:
- is the preview column collapsed?
- is padding_right on?
- is preview_files on?
- is preview_directories on?
- is the pager opened? ("i" key)
- does the current file have a preview?

This commit should fix border drawing in all of the mentioned cases.
config/commands: fix ":find .."
config/rc.conf: change untag keybinding from "T" to "ut"

This makes it more consistent with other key bindings like v/uv, V/uV,
dd/ud, yy/uy, q/uq, mX/umX and makes "u" a more universal undo-hotkey
core.actions: fix a rare, silent exception

There was some code assuming that fm.thisfile is not None.