(Option to) Make editor tabs fixed width

[elpres]

At the moment, every open editor tab's width depends on the filename. This makes closing several tabs in a row with the mouse cumbersome because after clicking the Close button on one tab, the next one's is in a different place, requiring the user to move the mouse to the new position. If the tabs would all be equally wide, one could click the Close button on the left-most tab of the ones one wants to close, and the next tab's Close button would appear at the same position. So a simple click would be enough to close it, no repositioning of the cursor necessary.