intitial connection from connection manager should be in its own thread

[mispp]

when initiating connection to any database, connection is established by GUI thread. i've tried connecting to a database behind vpn and, for a brief moment, gui freezes.
'initial' connection is later cloned for each tab in a new thread, so there it works fine.

this initial conneciton should also be handled in a separate thread, which would also be used to pull objects from the database once treeview control is implemented.

[mispp]

i guess with this change, GuiStateHandle class should be made, so based on all events coming, gui would be updated.

example:

• export data button should only be enabled when we a conneciton in a tab is active, when there is already executed query and execution of it is finished and when query is a "select"
• run query shoudl be enabled only when active connection is selected in a tab
• while there is a running query, connection switch should not be allowed, if needed, first the query should be cancelled.
• etc.

once treeview control is implemented, then also loading objects from database should be handled in a queue so the queries (forward only) don't get mixed up.