Make filter controls "sticky"

[MetRonnie]

Describe exactly what you would like to see in an upcoming release
Pin the filter controls to the top of the lumino tree & table views so they don't scroll out of view.

From Hilary on Element:

Anyone noticed this cylc-ui regression?

We merged this PR late January: #861

As I recall it pinned the filter view toolbars to the top of the lumino tabs, for table and tree view - i.e. independent of view scrolling.

However, since then (and possibly for rc1 which was released not long after) the tool bars have become unpinned again.

Looks like the regression occurred on the PR itself. I just built cylc-ui on the first commit of that branch, and the filter toolbar was pinned to the top of the table view at that point.

Pull requests welcome!

[oliver-sanders]

Note: The search & filter controls (along with any other controls a view needs) are intended to be combined into a "view toolbar" - #471

This toolbar would be "sticky" at the top of the view.