Create and reload regular snapshots for long-running boards

[andrasfuchs]

We already have a snapshot function with the -im command line argument, but it's not well documented and rarely used.

It would be great if it worked seamlessly in the background, making a restorable snapshot every ~30 minutes. This would help projects like @lynchaj Z80 manageable even if the routing process gets terminated unexpectedly.

Here is an example file for testing.

[andrasfuchs]

This feature needs to be broken into smaller tasks:

• [x] Get the platform specific location of the temp folder
• [x] Calculate the CRC32 checksum value of the input DSN file after a successful load
• [x] Check if there is a file saved with that CRC32 in the temp folder already
• [x] Confirm with the user and load the most recent snapshot from there
• [x] Change the -im command line argument to a -im <period> where we can define the snapshot period in minutes
• [x] Save the most recent design as a binary file periodically using their CRC32 in their filename
• [x] Remove the snapshot after a completed autorouting session
• [x] (optional) Change the file extension to FRB (freerouting board)

[andrasfuchs]

It still needs some testing, but I implemented this feature. It will be available in the next release.

Every time a better routing is found, a new snapshot is saved. If the routing is cancelled by the user or terminated in any other way, the next time we open the original file the above confirmation dialog offers the continuation of the process by loading the snapshot file back.