This method fills out the passed-in BList of modified utf-8 characters for
a given utf-8 character and set of modifiers.
For example if you pass in "=" and B_SHIFT_KEY the list will get filled
out with each character in the shift map that has "=" in the normal map.
Each supported keymap modifier combination is available.
The reason this is useful will soon become apparent.
A BList is used because the character might be mapped multiple times,
for example if you have a Mac keyboard you've got two "=" keys, one in
0x1d and one in 0x6a.
The caller is responsible for creating the BList and destroying it as well as
freeing the resulting character strings.
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