Add handler for files

- Retrieve file URLs from the wiki API.
- Load titles in the `File:` or `Image:` namespace as files.
- When a file URL was received, open the image dialog to display it.
Remove image select handling

File links will be opened with the default link opener.
Don't remove links on images

Image links to other places should be respected (using
`[[File:XY|link=target]]` syntax. Image links to the file descriptor are
handled by the default link handler.
Show spinner while loading file information
Remove duplicated code... (I should've known it...)
Refactor file URL loading

Load file information simultaneously with the normal title loading. For
non-File titles, no additional data is shown. All File titles are within
namespace 6 and can be identified unambiguously without checking for all
possible localizations of the File namespace prefix. In case of
namespace 6 (a File title), image information is included in the API
response and the data can be used to start the image display instead.
Remove further log output