prefs/directories: surface that the Incoming folder is shared (#601) - #631

…ject#647)

Fixes amule-project#631. Opening a collection from a file manager has never worked on any platform: nothing declared the type to the OS, nothing handled the open-document event on macOS, and the command-line path rejected anything that was not an eD2k or magnet link.

Accepts a .emulecollection path or file:// URL as an argument, handles MacOpenFiles on macOS, and declares the type to each OS. Registration is exposed on the same four surfaces as the URL schemes so a portable copy can register itself; UriScheme becomes HandlerTarget so the existing policy layer and the Linux mimeapps.list handling are reused rather than duplicated.

Hardens CMuleCollection, which becomes reachable from arbitrary files: a blank line threw std::out_of_range out of Open(), a length prefix could request a 4 GiB allocation, a filename containing a newline forged extra lines in the ED2KLinks IPC file, an entry without a hash produced an unusable link, a link smuggled into a filename was harvested by the text parser after the binary parser rejected it, and a UTF-8 BOM made a collection parse as empty. Adds unit tests, which the parser had none of.

Also fixes pre-existing bugs this feature was the first to depend on: wxExecute's string form passed quote characters through to the XDG cache helpers, those helpers inherited the AppImage's LD_LIBRARY_PATH and died on a bundled glib symbol, mimeapps.list was written without a trailing newline so the next writer's entry fused onto ours, and four installer strings were never extractable by xgettext.