Change deadkey behavior

[dylanhand]

The deadkeys are working. I am not sure I agree with the way bidirectional dead keys are implemented (I notice I have added this myself to FAOo). The way it is now, one basically fakes such dead keys by defining regular letters as the dead key, giving the desired output when followed by the expected accent.

What I think should be done instead is:

• define the corresponding combining diacritic somewhere
• when typed, the combining diacritic automatically combines with the preceding character, giving the desired character, but in an undesired, de-composed form
• do unicode normalisation into precomposed form on the resulting character, and:
  • if the result is among the expected dead-key outputs, return that character
  • if the result is NOT an expected dead-key output, return the base char + the corresponding non-combining diacritic

This shifts the burden from the linguist to the app, for things that for most users are identical things, and also makes the layout .yaml files much cleaner.

These are not first priority changes, we can release the present version as soon as the crashing bug has been fixed. After that we can return to this issue.

Other issues related to dead-keys that should be looked into going forward:

• give better feedback that something is a dead-key, perhaps along the lines of rendering them as follows: ◌̀
• after pressing a dead-key, highlight possible continuations on the keyboard

Originally posted by @snomos in #225 (comment)