Support mouse wheel to scroll thru images.

Warning: this adds API 12 dependence.
Add rough support of zoom type switch on double-click.

This doesn't work to well as it is - zoom change doesn't happen immediately,
and actually happens for few images after one on which click happened. But
event handling is there at least.
When opening a directory, don't ask unneeded questions.
Add workaround for thmbnail flickering when starting PhotoView activity.
Make thumbnail updates when scrolling go from first to last, not reverse.

What we want to achieve here is:
1. Once user scrolls, we want to start showing thumbnails for items
visible after scroll ASAP.
2. We want to show them from beginning to end, not the other way around
as was in original ComicsReader.
3. But first line may be almost completely obscured, so no actual thumbs
is visible on it (only text titles), so we want to skip them.
4. Or first line may be only partially obscure, and small part of thumb
may be visible. In this case, we want to start with fully visible lines,
but get back to first partial line in the end.
Add PhotoView-based image sequence browse implementation.

https://github.com/chrisbanes/PhotoView

To get mouse wheel support, use https://github.com/pfalcon/PhotoView fork.
Add PhotoViewActivity, comment out old ViewerActivity.

They actually can remain both, and user will be offered both to view
images by intent type, both will work, but that's confusing.
Add PhotoView reference and bump target API.

android-12 is minimal API to use mouse wheel.