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.h

@@ -81,7 +81,7 @@ std::string string_unwordwrap(const std::string& s);
 
 bool glyph_dimensions(uint32_t flags, uint8_t* glyph_w, uint8_t* glyph_h);
 
-int len(uint32_t flags, const std::string& t);
+int len(uint32_t flags, const char* text);
 int height(const uint32_t flags);
 
 void print(