Add Linux Support

[BenHerbst]

I now used Pixel Studio for more than a half year and I can't be happier with it. It has everything I need to get my work done and more. In the time I switched from Windows to Linux as all my tools are also avaible there like Godot and I do drawing on my Android tablet, but sometimes I want to do just a small fix on some sprites like not scaling correctly and then there is no pixel studio on my Linux, so I have to move from Laptop to Tablet, open the file there again and fix it. There might also be users that just use Linux and have no Android or iOS tablet and want to use Pixel Studio, and I don't know but I think as it is build with Unity it won't be such a big deal, and it could be published via Steam, as it also features buying etc. or via Flatpak or snap, but most would prefer Flatpak as it runs everywhere. And yes, Pixel Studio is not open source and some Linux users would only use open source software, but most of them also use non open source software.