Optimize memory usage of UI flows

[bigspider]

in display.c, instead of using a union large enough to contain all the strings of the biggest UX layout, we could use the approach used here with UX_STEP_NOCB_INIT to limit the global memory usage to largest strings used in a single UX step.
That should free up over 128 bytes of RAM.

(thanks @xchapron-ledger)

[bigspider]

Refactoring attempted in 466daeb, 212360c and 9f3bfbd; doesn't quite save a lot of memory, although it might still be worth doing as it would likely simplify the UI flows and the g_ui_state structure.

907176a shows that a more ambitious approach of keeping the flows entirely on the stack when they are being used (and getting rid of g_ui_state altogether) would be possible.