Improve account button for multiple accounts

Currently, if multiple accounts are set up in Gajim, they are displayed stacked on top of each other in the account side bar to the bottom left. There is no limit, the account side bar grows indefinitely. To improve this situation, and to add convenient features to it, the following approach seems to be best:

If a single account is enabled:

• account's avatar is shown (including status circle of this account)
• left click opens the account's page
• right click opens a status selector for this account's status

If more than one account is enabled:

• a generic avatar icon is shown (status circle shows global status)
• left click opens a popover listing all accounts; clicking an account opens the account's page
• right click opens a status selector for global status

This way, the global status selector in the app page (top left button) can go away.

Connected to #11677, where notifications (e.g. subscription requests) would be shown.