Internal: Miscellaneous refactorings

[Taitava]

(The title can be improved/clarified later).

This issue is a collection of some internal code refactorings I'd like to do at some point. The list is not exhaustive, I'll add more stuff along the way.

• [x] Convert (almost) all let variables to consts. Ofc do not convert variables whose value changes. (The term variable does not mean SC's {{variables}}, but just normal programming variables instead).
• [x] Make sure all class members (= methods and properties) have access level defined explicitly: private, protected or public.
• [x] CSS: Rename all classes that start with shell-commands- so that the prefix is shorter: SC-.
• [x] Many class names are prefixed with ShellCommands. Shorten most of the class names by renaming ShellCommands to SC_. E.g. ShellCommandsPlugin -> SC_Plugin.

[Taitava]

All done so far, unless some new general refactoring ideas come to my mind. 🙂