Add `SLIsTrueWithTimeout` and deprecate `SLWaitUntilTrue`.

[wearhere]

To resolve confusion between SLWaitUntilTrue and SLAssertTrueWithTimeout (#236), SLWaitUntilTrue should be renamed to SLIsTrueWithTimeout. This will be done in two steps.

First, in the 1.2 timeframe, SLWaitUntilTrue should be deprecated by duplicating it as SLIsTrueWithTimeout, switching Subliminal to use it internally rather than SLWaitUntilTrue, and causing SLWaitUntilTrue to log a warning.

Then, in the 2.0 timeframe, SLWaitUntilTrue should be removed: #242.