Never emit the fileMiddleClickActivated signal if isTabsForFilesEnabled is true

Commit d27ee07d makes it impossible for applications embedding Dolphin part (for example, Konqueror) to react to middle mouse click on a file. If I understand correctly (I'm not familiar with Dolphin code) the fileMiddleClickActivated signal is connected with a slot in DolphinViewContainer, which is only used by Dolphin itself and not by DolphinPart. The result is that middle clicking on a file (except archives) from a Dolphin part has no effect.

To avoid this situation, I removed the check for !archiveProtocolIsEmpty() in the else if condition. This way, if isTabsForFilesEnabled() is true, which should be true if and only if the view is inside a DolphinPart, the activeTabRequested or tabRequested signal will be emitted, allowing the embedding application to respond however it wants. When the view is inside the Dolphin application, instead, isTabsForFilesEnabled() will always be false, so the new behavior will be used.