Let the TouchHandler::GestureGet() function return a TouchEvent instead
of the touchpanel-driver specific enum.
This helps to move the driver specific helper function `ConvertGesture`
from `DisplayApp` into `TouchHandler`.
Don't use relative imports like `../foo.h` as those depend on the
relative position of both files. Rather than that use imports relative
to the `src` directory, which explicitly is part of the include
directories.
