Page turner progress bar should move leftward for RTL books

**Description**

(With much appreciation to @Pyrojet99 , @iisa, and others I don't know of who have worked to make RTL reading a good experience! ... ) Although the arrow buttons, arrow keys, and page turning animation all match the reading direction for right-to-left books, the progress bar on the bottom does not. It moves _rightward_ as the reader progresses through the book _leftward_.

**Evidence / Screenshot (if possible)**

From https://archive.org/details/gendaitankashu00meijuoft/page/27/mode/2up

**Expectation**

For RTL books,
- the progress bar should begin on the right for page 1 and move leftward as page numbers increase
- dragging the progress bar handle leftward should increase the page numbers, rightward should decrease them
- clicking to the left of the progress bar handle should jump to a later page, clicking to its right should jump to an earlier page

**Context**

FF 84.0 on Mac 10.14.6

**Proposal & Constraints**

?

**Stakeholders**

@right2leftdh