Move config transforming of to Qt values to configtypes.

This is a preparation for #46.
Add --left/--right/--opposite to :tab-close.

This fixes the updated version of #46.
Regenerate settings.asciidoc after renaming last-close value.

This wasn't done after 3e31e88/#154.
Add a cmdutils.check_exclusive function.
Update outdated comment
Add --left/--right arguments to :tab-only. Closes #46.