Make MarkText a Portable app

[MarDiLe]

for those like me, that want MarkText to use for work but restricted to install on a company computer. please create a portable version.

[rail5]

Hey, I'm not a dev on this project but you might be helped by the following:

• For GNU/Linux: AppImages are included in the release page. Here is the portable AppImage for v0.17.1
• For Windows: Releases include what appears to be a .zip file of MarkText in portable form. v0.17.1 for 64-bit and for 32-bit
• For MacOS: Apparently portable versions for x86_64 and for Apple M1

Granted, I don't use/fully understand Windows or Mac, so I'm not 100% sure, but it looks like those releases are portable. At any rate, the Linux AppImage definitely is.