Roster model proxy

It's time for roster. We want to show multiple accounts in one window. Hence we decided to make every active account being a top level element of roster and thereby downgrade group elements.

I have implemented proxy model that makes use of current RosterModel. It contains collection of <RosterModel, Account> pairs. For now, we can show one account contacts but code is more prepared to jump into multiple view.

Account serialization

We wanted profile settings to be stored using boost::serialization. It's done now for Account class (including ClientOptions serialization and other) that remember profile details. Saving and loading objects is realized in AccountsManager.

Previously, account details were stored using ProfileSettingsProvider. Now, for backward compatibility reason, they're stored in both settings provider and serialized string. When no serialization string is detected in settings, Swift obtain details using current settings provider and stores them serialized also.

Places in code using settings provider were replaced to make use of Account serialization. In future, we are probably going to avoid using ProfileSettingsProvider at all.

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).