[sidebar view] Fix sidebar header alignment and height

The search field's header has a totally custom view. This custom view
did not make any attempt to match the height of adjacent headers, nor to
vertically align its contents, relying on items' sizes never changing.
As a result, the header could become mis-aligned with adjacent headers,
and the search field had unequal top and bottom badding with certain
font sizes and scaling settings.

This commit fixes these issues by explicitly handling those cases.

It also does the same thing for the subcategory header to make sure they
match.


(cherry picked from commit 2c180c62)