produceOptionNameByIndexCallback for GEMSelect and GEMSpinner

[Spirik]

• Support for produceOptionNameByIndexCallback() callback that is called before option name of GEMSelect or GEMSpinner is printed on the screen (allows to override option name being printed, e.g. to form it dynamically based on index of an option, or modify to add ellipsis character for overflowing values, etc.);
• GEMSelect::setProduceOptionNameByIndexCallback(), GEMSpinner::setProduceOptionNameByIndexCallback() and GEMSelect::removeProduceOptionNameByIndexCallback(), GEMSpinner::removeProduceOptionNameByIndexCallback() methods;
• ::getMenuItemValueLength() method made public (returns length available for menu item value to fit on screen);
• ::getMenuItemTitleLength() method made public (returns length available for menu item title to fit on screen);
• GEMSelect::getType(), GEMSelect::getLength(), GEMSelect::getSelectedOptionNum(), GEMSelect::getSelectedOptionName(), GEMSelect::getOptionNameByIndex() methods made public;
• GEMSpinner::getType(), GEMSpinner::getLength(), GEMSpinner::getSelectedOptionNum() methods made public;
• GEMSpinner::optionNameToCharBuffer() method to copy option name of the supplied GEMSpinnerValue object to provided char* buffer;
• GEMSelect::setOptions() method to specify another set of available options (e.g. to support different option names for different languages);
• New detectEncoder() function in supplied examples with rotary encoder that makes compiled sketch size a little smaller and loop() easier to read.