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Previously, on real hardware (emulator behaved fine), we would routinely
get repeat keys unless pressing the button very fast. This commit solves that.

Previously, when being in A-Lock mode, activating Alpha wouldn't make
us go temporarily in digit mode, as is expected.

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no a-lock or lowercase yet.

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no alpha yet.

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For now, only digits are scanned. Lifted from my "tiseg7" example.

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This was mostly lifted from my "tihello" example, but it required significant
adjustments.

This commit also introduces a font management system. A lot of fonts are
available online, but sources aren't always clear so to avoid copyright
landmines, I re-created my first 5x7 font from scratch.

As it is now, this resulting ROM gets "Collapse OS>" to be displayed on the
LCD screen. Much work still left to do.

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