keyboard: replace CcAltCharsKeyDialog with a generic XkbModifierDialog

Previously, there were multiple redundant lists of possible values for
this setting. This uses one struct to define not only all the possible
values, but also anything specific to the alternate characters key, so
this can be used for other modifiers.

This also changes the style of the dialog to use a GtkListBox.
7 jobs for XkbModifierDialog in 3 minutes and 53 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Build
passed #906376
build

00:03:00

 
  Test
passed #906377
test

00:00:53

 
  Manual
manual #906379
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asan
manual #906378
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flatpak
manual #906382
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msan
manual #906380
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tsan
manual #906381
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ubsan