Behavior for suggestions of incomplete (and complete) words

[aarppe]

... selecting a suggestion that ends with a character marking an incomplete word [e.g. the |-sign] should not be appended with a space character (as the word yet needs to be completed) ... .

Originally posted by @aarppe in #13 (comment)

... but selecting a completion suggestion that does not end with a character marking an incomplete word (e.g. does not end with the |-sign or some other character used to indicate this) should be appended with a space character.

This behavior could either be implemented in the mobile speller FSTs (in which case the code should not append any space character in any event), or then it could be implemented through the code.

Whichever way we go depends on what we end up deeming as the best way for the keyboard to behave in indicating the incompleteness and completeness of both the input and suggestions.

Subtasks:

• including the incomplete word marker (currently tilde: ~) among word-internal characters (Treating the marker of incomplete words as a word-internal character #13);
• specifying that a space is not output at the end when a suggestion which is an incomplete word is selected; and
• thinking through whether a space should be output when a suggestion which is complete word is selected (since sometimes such a complete word is a prefix of another complete word). It would seem that the default expectation would be for a space to be output after selecting complete words, though then one would need to delete that if such a complete word is also part of a longer word.

[aarppe]

Changed the marker of incomplete words to tilde ~.