Another language switching. Intermediate.
Switching of translations in runtime for MainWindow
RU translation
retranslateUI method for ExplorerTreeItem
ExplorerWidget: retranslating items
Retranslating of ExplorerCollectionTreeItem
Retranslating of ExplorerDatabaseCategoryTreeItem
Retranslating of ExplorerDatabaseTreeItem
Retranslating of ExplorerTreeItem
Retranslating of ExplorerServerTreeItem
Retranslating of ExplorerUserTreeItem
'System locale' language fixes