Add free space label to main window statusbar

[keizie]

Description

To fix #246, add free space label into bottom status bar

Changes made

• mainwindowstatusbar.* - add label widget and set text with other status labels
• serverstats.* - add freeSpace setter and getter
• rpc.* - getServerStats query freeSpace too

Additional Information

• serverStats used to relay freeSpace value to setText inside mainwindowstatusbar.cpp QObject::connect, as Rpc::getDownloadDirFreeSpace is coroutine. I don't know how to handle co_await inside QObject::connect.
• It takes number of seconds until freeSpace number updated, after debug log shows free-space call succeed. I think I somewhat missed "update right away" code.

[equeim]

Thanks!