Support __FlashStringHelper for other string based methods

[Jnesselr]

I think it would be useful if other string methods supported __FlashStringHelper to be able to read directly from PROGMEM.

The methods that maybe should have a PROGMEM option:

• WriteChars
• file_Count
• file_Dir
• file_Erase
• file_Exec
• file_Exists
• file_FindFirst
• file_LoadFunction
• file_LoadImageControl
• file_Open
• file_PlayWAV
• file_PutS
• file_Run
• gfx_Button
• putstr
• writeString
• file_FindFirstRet (only for Filename)

Unfortunately, I can't see a good way of doing this that doesn't involve lots of code duplication. It's more or less what happened for the println/print function. I don't mind writing the code and submitting a PR, I just wanted to get some clarification on whether it's the right approach to duplicate these functions. For all of them but WriteChars, I think they just pass into WriteChars. So the only function that would actually be different is WriteChars itself, every other function would be identical aside from type info.