Add support for toneless key in ZhuYin keyboard layout

[mrubli]

#51 added support for "toneless" (incomplete) input for the PinYin keyboard layout. As discussed there it would be great to add the same feature for the ZhuYin keyboard layout.

In the ZhuYin layout the space key is used to input first tone, so using that would add ambiguity to the input. Other platforms' IMEs solve the problem of users not knowing the right tone by letting the user choose a so-called "toneless key" in the preferences, typically something like \ or =. \ is rarely ever used in writing and = is often used in the context of maths/numbers where many people switch to English mode anyway, so the potential for conflict/confusion is minimal, especially when typed in context.

This animation shows ㄇㄚ with third tone, second tone, and finally in toneless mode. In this example the toneless key was set to =. (The last frame merely illustrates the expansion of all possible intonations using the cursor right key.)

To simplify things we could start off with a single hardcoded key (possibly with a boolean option to enable/disable its use), and eventually extend it to allow people to choose from a list of keys. The latter would be very desirable for people who switch back and forth between different systems or have year-long habits of using a specific key already.

As mentioned in #51 I'll try to write up with a prototype for this, so this issue here is mainly for tracking progress.