Bug 431201

SUMMARY
I run an older master appimage recently and I got the "an update is available!" button, so I clicked it out of curiosity, and it actually worked (although I had to install gpg2 to make it work: should I report it separately? It did tell me to install it so there was no issue, but...).

However, I couldn't just go and open the next appimage, since it wasn't marked as executable. I know this is normal for appimages or any files downloaded from the internet, but since I was updating a file that was marked executable and I executed it, maybe it would be possible to make it easier for users and mark the new one as executable automatically?

If it's important to not do stuff like that without user's permission, what about a checkbox? It's still easier to check one than (1) right-click, go to Properties, go to the correct tab, check the checkbox, click OK; (2) right-click, use "Open in Terminal", write `chmod +x krita...`, make sure you mark the correct appimage, then start it (also making sure you use the correct appimage - with a lot of appimages, Tab doesn't help much).

STEPS TO REPRODUCE
1. Make sure you don't have the latest appimage from the binary factory in the folder and that you have gpg2 installed.
2. Run Krita appimage.
3. Click "Update now" on the Welcome Screen.
4. Close Krita, try to start the new appimage.

OBSERVED RESULT
It doesn't start because it isn't marked as executable.

EXPECTED RESULT
It starts with no issues.

SOFTWARE/OS VERSIONS
Krita

 Version: 5.0.0-prealpha (git a1582db)
 Languages: en_US, en, en_US, en, en_US, en, pl_PL, pl, pl_PL, pl
 Hidpi: true

Qt

  Version (compiled): 5.12.9
  Version (loaded): 5.12.9

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 5.3.7-050307-generic
  Pretty Productname: Linux Mint 19.3
  Product Type: linuxmint
  Product Version: 19.3
  Desktop: X-Cinnamon