Add ability to cancel in-progress compression

[imathrowback]

Add a cancel button to cancel in progress compression without leaving behind processes.

If the window is closed while a compression is running, the compression child process is left running.

[NoFunk]

Can confirm, ran 5 compressions then closed them all, the processes are still open in the background.

[Iridium-IO]

There's an issue with this - I can't get the background process to gracefully shut down when it's called from within CompactGUI (WTF Microsoft).

For now what I've done is added a check: If an active compression/decompression is running and you try to exit, it will show you a warning telling you that proceeding with closure is potentially dangerous. If you then decide to continue, it will forcefully kill the process, potentially leaving a semi-compressed file behind.
You can repair this semi-compressed file by letting the program run with the Force compression/uncompression option selected, then either select "Compress Folder" or "Uncompress Folder"

[imathrowback]

A reason to have a cancel option is when you go to compress a directory full of gigabyte files (eg, game resources) and and if they don't' compress well, you don't want to continue compression as you will get nothing out of it and it'll take ages to complete.

An alternative fix might be to call compact.exe per file rather than using the /s command.

[Iridium-IO]

Yeah I'm looking into that as an alternative, but I don't like the idea of taking more control away from the Windows command. I'd prefer to leave as much as possible up to Windows to do, since this is meant to be a visual interface - nothing much more.

Right now if you look at the wiki you can see which games compress well and which don't, which should help with avoiding most of the big ones.