Improve roster management code

Use account switcher window to select context for actions like changing
status or adding contacts. Moved "Settings" from account switcher to
roster menu as "Accounts" (like in pre-0.3 builds). Add context support
for removing and renaming contacts. At least VCards and subcsriptions
might still be broken.
Fix scalling of unread count indicator

In case list element size was increased to fill the whole status text,
unread message count would be scalled improperly.
Start experimenting with something

Added another list model without contacts which are offline o save
memory and improve rendering when "hide offline contacts" feature is
enabled.