TextEdit: Apply filters to restrict entry

Allow the textedit to take a nk_plugin_filter to restrict the kind of codes it can take. Like...

NK_API nk_bool nk_filter_default(const struct nk_text_edit*, nk_rune unicode);
NK_API nk_bool nk_filter_ascii(const struct nk_text_edit*, nk_rune unicode);
NK_API nk_bool nk_filter_float(const struct nk_text_edit*, nk_rune unicode);
NK_API nk_bool nk_filter_decimal(const struct nk_text_edit*, nk_rune unicode);
NK_API nk_bool nk_filter_hex(const struct nk_text_edit*, nk_rune unicode);
NK_API nk_bool nk_filter_oct(const struct nk_text_edit*, nk_rune unicode);
NK_API nk_bool nk_filter_binary(const struct nk_text_edit*, nk_rune unicode);