Improvements to forward/back mouse button handling in file widgets

This commit modifies KDirOperator so that:

• moves the handling of forward and backward buttons to MouseButtonPress, bringing behaviour in line with Dolphin, and stopping the ability to drag items using these buttons (mentioned bug #443169)
• also makes that case handle MouseButtonDblClick, so the buttons can be pressed in quick succession without being swallowed
• makes that handler discard the event, preventing single-clicks from selecting items on the new page, and double-clicks from opening files or folders under the mouse

There is still an edge case with dragging, in that it is still possible to drag an item with the forward/back buttons if there is no more history and the user selected the item via left-clicking first. I wasn't able to figure this out without an ugly hack (checking for the buttons in KDirModel::flags and disabling dragging). In my opinion though, that's much more of an edge case, and this commit should still be an improvement.