Add check_sketch_structure function

[per1234]

• Correct case of sketch filename extension.
  • Recognized extensions are .ino and .pde.
  • Only that extension case is recognized on case-sensitive OS like Linux.
• Folder name must match file name of one of the sketch files (.ino, .pde) it contains.
  • Folder/filename case mismatch causes move dialog when opening the sketch on filename case sensitive OS like Linux (verified).
• Valid sketch name.
  • According to the Arduino IDE: "Sketch names can only consist of ASCII characters and numbers and be less than 64 characters long."
  • No spaces.
  • Starting with a number is not allowed with Arduino IDE 1.8.3 and older (OK with 1.8.4 and newer).
• Only one sketch file that contains void setup() per folder