#The Tweetdeck interface to Twitter It is available at tweetdeck.twitter.com, and does everything you've asked for:

###Runnable under Microsoft Windows 8 Yes, works in any modern browser on Windows 8

###Ideally runnable on Windows 10 too Yes, works in any modern browser on Windows 10

###Aware of multiple accounts (whether if I need to change the current user scope or if I'm able to select which I want to use for the next tweet) Yes, you can have columns for multiple accounts, choose which accounts you tweet from, which you reply from, which you DM from, which you Follow from.

###Show all Twitter related stuff like mentions, messages, etc. Yes, you can have columns for each of those

###Metro awareness is nice to have Yes, If you have a metro browser, it should work in that. It works in the Edge browser.

###Ideally actively developed, as I won't change the application after the next API change Yes, it's actively maintained and developed by Twitter

The Tweetdeck interface to Twitter

It is available at tweetdeck.twitter.com, and does everything you've asked for:

Runnable under Microsoft Windows 8

Yes, works in any modern browser on Windows 8

Ideally runnable on Windows 10 too

Yes, works in any modern browser on Windows 10

Aware of multiple accounts (whether if I need to change the current user scope or if I'm able to select which I want to use for the next tweet)

Yes, you can have columns for multiple accounts, choose which accounts you tweet from, which you reply from, which you DM from, which you Follow from.

Show all Twitter related stuff like mentions, messages, etc.

Yes, you can have columns for each of those

Metro awareness is nice to have

Yes, If you have a metro browser, it should work in that. It works in the Edge browser.

Ideally actively developed, as I won't change the application after the next API change

Yes, it's actively maintained and developed by Twitter