Update ua switch

[taobert]

Uaswitch no longer loads (and prevents the browser from starting). This PR updates luakit-plugins/uaswitch to use modes.add_cmds, settings rather than globals and the new _M style modules. (I'm assuming this is in line with #15, but that issue's light on details.)

A few things to pay extra attention to (I'm obviously happy to change anything): - ua_strings.lua returns its table rather than assigning it (as _M can't be seen). - A level of indirection is removed on startup: load() which just called load_ua_strings() is removed and load_ua_strings() called directly. - A level of indirection is removed with the default ua handling: ua_alias_default is removed and "default" used explicitly. (As there was no benefit to having a variable.)

I also have the (2017 era) list of UAs from #16 staged and ready to commit, if you'd like those included in this. (I assumed since there was a separate issue, a separate PR were appropriate).

I also have an enhancement to call a menu to display known UAs staged and ready to commit.