Revise Qt Multimedia usage

[KitsuneRal]

Although Qt 6.2 finally has got Qt Multimedia, libQuotient cannot never could make proper use of it. To hold the
dependency just for the sake of extracting metadata from an audio/video file in one particular place looks like a waste; and without a GUI it is rather challenging to provide any added value on top of that. Moreover, the RoomMessageEvent constructor accepting a QFileInfo (the only part of Quotient actually using Qt Multimedia, and even that uses literally one method, QMediaResource::resolution, for the video image size) is not extensible: it doesn't support adding new kinds of content which will become a shortcoming once extensible events land in Matrix.

Therefore I think of deprecating this constructor if compiled with Qt 5, and entirely dropping it if compiled with Qt 6 (building with Qt 6 is an experimental configuration anyway for now; and its Qt Multimedia API doesn't allow to read even that image size value synchronously anymore). Clients will have to create an appropriate EventContent object themselves. It's likely that they already have everything necessary to extract content metadata if they provide any sort of audio/video/image preview; and even if not, it boils down to making a round over an event loop to read the metadata from the media file.

This would also lower the Qt 6 requirement to from 6.2 to 6.0, although versions before 6.2 are immature and aren't expected to be used much anyway.

Update: after moving around the code I remembered why I introduced both Room::postFile() and that RoomMessageEvent constructor in the first place: the current API to construct event content objects is quite low-level and desperately calls for something simpler on top of it. For example, it doesn't protect from a very easy mistake to pass the full path instead of just a file name to originalFilename, leading to disclosure of the local file structure to Matrix. While it may be worth it to retain the low-level API close to what it is now (with QMediaPlayer as an additional source of metadata when they are immediately available), Room could provide asynchronous means to load media metadata from a file by creating that QMediaPlayer instance and waiting on availability of the metadata before uploading the file and constructing a message event for it.

[CarlSchwan]

Make sense also QMediaResource::resolution didn't really work well for in my experience

[KitsuneRal]

Yes, that too.

[KitsuneRal]

9a5fa62 broke the build with Qt 6; stay tuned.

[KitsuneRal]

...and, while fixing the build breakage, I recollected the reason for the current code. I think Qt Multimedia is here to stay; Quotient just has to make better use of it.