Add an AppTab enum and rename ConfigTOML to ConfigToml

[gondolyr]

Purpose

This PR adds further readability and maintainability improvements.

Summary

• Add an AppTab enum to track the active tab.
• Rename ConfigTOML to ConfigToml to conform to Rust RFC 430.

  In UpperCamelCase, acronyms and contractions of compound words count as one word: use Uuid rather than UUID, Usize rather than USize or Stdin rather than StdIn. In snake_case, acronyms and contractions are lower-cased: is_xid_start.

• Getters have been renamed to not use the get_ prefix as recommended by the Rust API Guidelines.

  With a few exceptions, the get_ prefix is not used for getters in Rust code. [...] The get naming is used only when there is a single and obvious thing that could reasonably be gotten by a getter. For example Cell::get accesses the content of a Cell.

• Clippy's suggestions for the queue module's remove() function have been applied.

  ❯ cargo clippy
  warning: unneeded `return` statement
     --> src/helpers/queue.rs:182:35
      |
  182 |           if self.items.len() == 0 {
      |  ___________________________________^
  183 | |             return;
      | |__________________^
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#need
  less_return
      = note: `#[warn(clippy::needless_return)]` on by default
      = help: remove `return`
  
  warning: length comparison to zero
     --> src/helpers/queue.rs:182:12
      |
  182 |         if self.items.len() == 0 {
      |            ^^^^^^^^^^^^^^^^^^^^^ help: using `is_empty` is clearer and more explicit: `self.items.
  is_empty()`
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_
  zero
      = note: `#[warn(clippy::len_zero)]` on by default
  
  warning: `kronos` (lib) generated 2 warnings
      Finished dev [unoptimized + debuginfo] target(s) in 0.06s