Sidebar: Move final trailing separator out of main layout
tl;dr The scroll bars must not be overlayed with foreign items like frame borders, separators etc. They are doing fine on their own. Don't.
And since Kirigami.ApplicationItem was not designed for this kind of usage where you stick something permanent to one side, just more it out completely in its own layout. Components are refactored in different files, so that semantics of ApplicationItem being a QML context on its own is preserved, and properties, methods & signals are wired together to act as a proxy.
With Breeze and Kirigami patches applied, this MR make the following 1px difference:
| State | Before | After |
|---|---|---|
| "usual" category |
 |
 |
| "usual" sub-category |
 |
 |
| RTL category |
 |
 |
| RTL sub-category |
 |
 |