[Patch] Add action to close all but the active tab in image viever

[GerdP]

My session file contains many different geoimage layers, one for each day.
When working with some older images I often see far too many tabs in the image viewer dialog.

There is no easy way to close these tabs, I have to click the small x-icon for each tab, often I miss the x and activate the tab instead.
There should be an action which closes all but the active tab.

[GerdP]

Patch to implement shortcut Alt+Y to close all but the active tab

[GerdP]

too many tabs

[GerdP]

Note: I'll work a while with this patch to see if there are any drawbacks. For now, there is no button for the action and it is always enabled. If needed I may change that so that the user has to right click on a tab to see a menu with "close tab" and "close other tabs".