Main changes:
* Consolidate scrollbar setup code, reducing replicated lines of code split across disparate gui elements.
* Rename `m_Width` to `m_Breadth` as `m_Length`/`m_Breadth` makes more sense, particularly for when we (finally) implement horizontal scrollbars.
Minor changes:
* Correct some spelling/grammar in comments.
* Change uses of "scroll bar" and "scroll-bar" to "scrollbar" for consistency.
* Update years on affected `.cpp`/`.h` files.
* Some removal of unnecessary comments, courtesy of elexis' wip patch for #3038.
**Note:** The modifications to `mainmenu.{xml|js}` are for test purposes only and are not intended to be included when committed.
**Note:** I'm fairly inexperienced when it comes to c++ and so if I've made any rookie mistakes (or even not-so-rookie mistakes) I apologise. And tell me //gently//, please :wink:.