Add filename sanitization

[Caesarovich]

-> Fixes issue #1063 #1180 #1101 (And possibly more but I couldn't find)

Description

As discussed in issue #1063 there currently is an error if a file is sent on a device with a different system of the sender. This is due to different character limitations in files' names depending on Operating systems and partitions. For example:

On my Linux computer I can create the file <cool>.txt and send it to my other linux computer without issue. But if I try to send it to my Windows computer or to my Android phone, LocalSend will fail to save the file and fail the transfer with error code 500. This is because <> are illegal characters on Windows and FAT32 devices (Android).

This PR fixes that issue by providing a sanitization step before saving a file. Like here:

Future<String> digestFilePathAndPrepareDirectory({required String parentDirectory, required String fileName}) async {
  final actualFileName = legalizeFilename(p.basename(fileName), os: Platform.operatingSystem); // <- here
  final fileNameParts = p.split(fileName);
  .....

The sanitization is handled by the legalize library that I made for the occasion. So if any issues like this open up, they can be addressed upstream here instead. I'd be glad if users could test it across the many different OS they possess, I couldn't test it directly on all the possible different systems.

Changes

• Now includes the legalize: 1.2.1 dependency
• Filename sanitization in the digestFilePathAndPrepareDirectory function
• Filename check for the FileNameInputDialog and QuickActionsDialog widgets

[Caesarovich]

I've lowered the SDK requirements :)

[Tienisto]

Thank you!