Keyword "Set Browser Timeout": allow NONE or EMPTY to reset to the default

Nobody has claimed this yet.

In `Set Selector Prefix`, the argument `${NONE}` or `${EMPTY}` simply resets the prefix to the default (which is "empty").

=> I hereby want to request to adopt the behavior of `Set Browser Timeout` to behave the same way:

```
# not necessary anymore
${old}= Set Browser Timeout 2
# (do something)
Set Browser Timeout ${old}

# instead:
Set Browser Timeout 2
# (do something)
# I don't care what the old value was, just reset
Set Browser Timeout ${EMPTY}
```
PS:
Perhaps also allow this one?
`Set Browser Timeout default`

Start by locating the implementation of the `Set Browser Timeout` keyword and compare its handling with `Set Selector Prefix`, which already accepts `${NONE}` and `${EMPTY}`. Check the related tests or keyword entry points; done means resetting the browser timeout with `${NONE}` or `${EMPTY}` behaves like the default, with the optional `default` spelling clarified.

Written by the indexing model from the issue text.