Feat: ui.page_scroller

[evnchn]

Motivation

Addresses NiceGUI Wishlist (view)

Implementation

• As mentioned, QPageScroller is very similar to QPageSticky, so we copy over the code and the function signature from ui.page_sticky
• Then, we add on the 3 new properties, scroll-offset, reverse and duration, inheriting the default values of 300ms duration and 1000ms scroll-offset from Quasar.
• Reworked the documentation, because the old one was so messy and hard to read.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).

-> No pytest for ui.page_sticky so no pytest for ui.page_scroller too.