Allow detection/specification of combined modifier keys

I was trying to use matchbox-keyboard to make a flexible international keyboard, but the shift and alt modifiers don't work as I'd expect them to, and I've run out of modX modifiers (there being only 3) to be able to capitalize accented letters. For example:

  <key obey-caps="true">
    <default display="a"/>
    <shifted display="A"/>
    <mod1 display="á"/>
    <mod2 display="à"/>
    <mod3 display="ä"/>
  </key>

I can turn 'a' into 'A', 'á', 'à', and 'ä', but I'm out of luck for 'Á', 'À', and 'Ä' (unless I put these other variations on an entirely different key, which would be ugly!). Perhaps something like:

  <key obey-caps="true">
    <default display="a"/>
    <shifted display="A"/>
    <mod1 display="á" shifted="Á'"/>
    <mod2 display="à" shifted="À"/>
    <mod3 display="ä" shifted="Ä'"/>
  </key>

...would be great, as well as attributes alted and alt-shifted for even more flexibility. With those extra variations, one could even add Â, â, Ã, ã, Å, and å, all on the one key where they belong.

An even more flexible approach would be the way the MacOS keyboard works, where accented characters are a two-stroke process, first typing a combining accent mark, then the character to which the accent is to be applied.