ColumnView: Explicitly support RTL in separators; fix colorSet
Separators should use Window color set, because it makes them stand out more, and look organic on top of toolbars where View set is too pale and inconsistent.
Generally speaking, separators should not be child items of their column, but rather be interlaced between them. It could be a topic for another MR. But so far we are parenting them to columns, so this MR fixes their anchors and visibility rules to work better in RTL environments.
| (Theme) | Before | After |
|---|---|---|
| Dark |  |
 |
| Light |  |
 |
Note about System Settings screenshots:
- Previously (Before) it worked like this: subcategory's column was the leftmost (x==0), so its separator got hidden; meanwhile main (first) column had its separator shown, but it lacked mirroring & anchors, so it stuck on the left side within column.
- now (After) main page's separator is positioned on the right and thus it becomes invisible. What you see is the leading separator of the second (subcategory) column.
- there is one more pixel to be fixed in Breeze and another one in System Settings sidebar layout itself, since it rolls its own final trailing separator.
