Implement dark/light/default theme for viewer handlers

[juliushaertl]

Contributes to nextcloud/text#82

This PR adds the capability for mimetype handlers to indicate in which theme mode they run for their file displaying. The following 3 modes would be available:

• dark: Background is always dark (e.g. current style, used for images)
• light: Background is always light (e.g. Collabora doesn't offer a dark mode and has a bright interface so it can use the light viewer theme)
• default: Background adapts to the accessibility settings (e.g. text adapts to the main background color so the default mode will use the same color as the modal background)

The only thing I'm not entirely happy is that the theme is only available once the viewer has fetched the fileinfo (which might take a bit depending on the network and server). In order to avoid flickering between dark/light, I for now introduced an intermediate loading state where there is no mask background and just the loading spinner until the theme is available. (See screen recordings for how that looks) Though I'm of course open for other ideas how to handle that.

Peek.2021-10-15.09-44.mp4

Peek.2021-10-15.09-44-dark.mp4

[nimishavijay]

Looks good! The spinner isn't that much of a problem. It is visible properly only when there are less number of files like in the Photos app screen recording. But it is not much of an issue and this looks like the best way.
So looks good to me! 🚀

[skjnldsv]

/compile amend /