Make the page browser just another View

[hnesk]

At the moment the page browser (ocrd_browser.ui.PagePreviewList) is a hardcoded part of the UI, with a pre-set file_group and position in the UI.
It would be nice if it would behave the same as the other views, so that the user would be able to instantiate, configure and close it.
This opens the possibility to have more than one page browser showing more than one file_group at once, e.g. seeing the binarized and cropped versions of all pages next to each other.
Also an user selected file_group would avoid to restrict the view to preconfigured file_group names like 'OCR-D-IMG.*' as discussed in #7
This can be achieved by changing the page browser to extend and utilize ocrd_browser.view.View with an ocrd_browser.view.FileGroupSelector