Recompressed PNG assets

Losslessly recompressed the PNG assets using PNGOUT, PNGCRUSH or APNGOPT to save some kilobytes.

To verify that recompression was lossless, use Imagemagick's
```
compare -verbose -metric mae old.png new.png null
```

```
Old bytes | New bytes | Ratio	
1185	1170	99%	extensions/b2g/images/div_line_left@2x.png
1105	1095	99%	extensions/b2g/images/div_line_left.png
 252	 251	100%extensions/b2g/images/div_line_right@1.5x.png
1193	1170	98%	extensions/b2g/images/div_line_right@2x.png
1108	1094	99%	extensions/b2g/images/div_line_right.png
2229	1762	79%	extensions/b2g/images/document_bg.png
1328	1285	97%	extensions/b2g/images/icon_next_page@1.5x.png
1355	1249	92%	extensions/b2g/images/icon_next_page.png
1371	1324	97%	extensions/b2g/images/icon_previous_page@1.5x.png
1393	1307	94%	extensions/b2g/images/icon_previous_page.png
2278	2192	96%	extensions/b2g/images/icon_zoom_in@1.5x.png
1885	1729	92%	extensions/b2g/images/icon_zoom_in.png
2111	2057	97%	extensions/b2g/images/icon_zoom_out@1.5x.png
1785	1645	92%	extensions/b2g/images/icon_zoom_out.png
5069	4322	85%	extensions/b2g/images/spinner.png
10774	8190	76%	extensions/b2g/images/toolbar_background.png
 726	 698	96%	extensions/chromium/icon16.png
2569	2212	86%	extensions/firefox/icon64.png
2014	1712	85%	extensions/firefox/icon.png
 381	 268	70%	web/images/findbarButton-next.png
 371	 272	73%	web/images/findbarButton-next-rtl.png
 371	 272	73%	web/images/findbarButton-previous.png
 381	 268	70%	web/images/findbarButton-previous-rtl.png
9025	3612	40%	web/images/loading-small.png
1291	 340	26%	web/images/secondaryToolbarButton-firstPage.png
1188	 315	27%	web/images/secondaryToolbarButton-lastPage.png
3061	 414	14%	web/images/secondaryToolbarButton-rotateCcw.png
3066	 414	14%	web/images/secondaryToolbarButton-rotateCw.png
 454	 290	64%	web/images/shadow.png
2459	2418	98%	web/images/texture.png
 244	 210	86%	web/images/toolbarButton-bookmark.png
 512	 343	67%	web/images/toolbarButton-download.png
 237	 200	84%	web/images/toolbarButton-menuArrows.png
 417	 348	83%	web/images/toolbarButton-openFile.png
 353	 296	84%	web/images/toolbarButton-pageDown.png
 558	 462	83%	web/images/toolbarButton-pageDown-rtl.png
 344	 307	89%	web/images/toolbarButton-pageUp.png
 426	 310	73%	web/images/toolbarButton-pageUp-rtl.png
 491	 450	92%	web/images/toolbarButton-presentationMode.png
 474	 437	92%	web/images/toolbarButton-print.png
 503	 411	82%	web/images/toolbarButton-search.png
 302	 296	98%	web/images/toolbarButton-secondaryToolbarToggle.png
1221	 583	48%	web/images/toolbarButton-secondaryToolbarToggle-rtl.png
 349	 306	88%	web/images/toolbarButton-sidebarToggle.png
1193	 548	46%	web/images/toolbarButton-sidebarToggle-rtl.png
 300	 261	87%	web/images/toolbarButton-viewOutline.png
3036	 405	13%	web/images/toolbarButton-viewOutline-rtl.png
 211	 182	86%	web/images/toolbarButton-viewThumbnail.png
 143	 141	99%	web/images/toolbarButton-zoomOut.png
75092	51843	69%	SUM / AVERAGE