Public module access, smaller include and style fixes

• It's now possible to access USerial class from other modules. Headers were made public.
• Including the class is now faster without the windows.h header include in Serial.h.
• The windows.h header is not used anymore. The smaller Windows/MinWindows.h, from Unreal, is used instead.
• Some code and comment style fixes are in place.

Fix Serial class export to external code

The Serial class was missing the UE4DUINO_API macro that exports it for usage on code outside the plugin.
Also fixed initialization order of members in class's constructor.

The code is still compatible with UE 4.21, in case anyone wants to build for it. Just remove the engine version specifier from .uplugin.

Fix bug that prevented port opening.

Fix shameful bug that prevented port from opening because when I refactored the code to 4.20, I deleted the port number from the string that requests the port to Windows.

Thanks yuanny on the forums for pointing that out!

Updated to UE 4.20

FString::Printf usage updated to accomodate to UE 4.20 update.

HotFix for crash when using ReadLine with no data from Arduino

2.2.1 HotFix!

No more crashes when ReadLine is used and there's no data to be read in COM.

v2.2 - new utilities for byte conversion

2.2 Update!

What's new

4 new utility nodes for converting from and to bytes/int/float:

• BytesToInt — Convert 4 bytes in little-endian order into an Integer.
• BytesToFloat — Convert 4 bytes in IEEE 754 standard into a Float.
• IntToBytes — Convert an Integer value into 4 bytes in little-endian order.
• FloatToBytes — Convert a Float value into 4 bytes matching the IEEE 754 standards.