Flathub support

[DUB1401]

I made several changes to publish this application on Flathub, because I consider it the best SSH manager, and allowed myself to publish it.

List of changes:

• updated manifest with a list of permissions;
• updated build scripts;
• fixed a problem with the icon (it wasn't really square).

I know that the project has not been supported for a long time, but perhaps these changes will someday be accepted into the main repository.