Login window GUI

New login window GUI is implemented. At a glance, we can see the same, single account window but there's "Use multiple accounts" link that moves a user to a list of accounts.

Every account has its name, color and login details. We can change connection options by clicking at cogwheel. There's also certificate button next to password input (as in single account view). Remove account with red cross.

Final aim is to login to each account by enabling it (checkbox next to status egg). This way we can easily choose accounts that are also automatically logged in at startup.

In this phase we can log in using only one account as roster GUI is going to be implemented. Enabling given account by ticking its checkbox effects going to roster when connected correctly.

SettingConstants::MULTIACCOUNT_ENABLED option determines wheter to use this new, experimental feature (default: false).