Partial fix for the Windows platform of the emoji issue (#1360)

This fixes input of emojis to Fl_Input and Fl_Text_editor widgets under Windows with the emoji palette.
Most emojis have a Unicode point > 0xFFFF and therefore are encoded as a surrogate pair
by Windows which uses UTF-16. Thus, Windows sends 2 consecutive WM_CHAR messages to the
window and gives one member of the pair each time. After the second WM_CHAR message arrived
FLTK is able to enter the emoji in its text. Windows may also send "variation selectors" and
zero-width Unicode points when dealing with emojis. FLTK just skips them.
Windows also translates some Unicode emojis into 1 emoji + 1 other Unicode point: for example
"woman pilot" produces "pilot emoji" + "woman" unicode point. FLTK now handles
this gracefuly.

This fix also prefixes the windows class names with "FLTK-" under Windows to prevent
collisions with Windows-reserved class names. That fix is necessary for the emoji palette
to be usable in some scenarios. That fix is still under debate and may evolve in latter commits.

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