Change font::len text argument from std::string to const char*

See the previous two commits, a lot of the time we don't need
std::string objects to be passed to these functions because we already
have C strings.

   Commit 1/3: font::print_wrap
   Commit 2/3: font::print
-> Commit 3/3: font::len

desktop_version/src/Font.cpp

@@ -1147,13 +1147,12 @@ bool glyph_dimensions(uint32_t flags, uint8_t* glyph_w, uint8_t* glyph_h)
     return true;
 }
 
-int len(const uint32_t flags, const std::string& t)
+int len(const uint32_t flags, const char* text)
 {
     PrintFlags pf = decode_print_flags(flags);
 
     int text_len = 0;
     uint32_t codepoint;
-    const char* text = t.c_str(); // TODO no std::string
     while ((codepoint = UTF8_next(&text)))
     {
         text_len += get_advance(pf.font_sel, codepoint);