WIP: Browse.vue: improve pagination controls (alternative)

This attempts to make the controls in Browse.vue easier to access, even when a large list of operations requires scrolling.

(It is an alternative to the design proposed by !35 (merged).)

The main idea is to keep the textbox with the filter and the page status/buttons floating on top of the page:

To maximize code reuse, this also extracts the pagination buttons to a new component, PaginationControls.vue.