Fix the tab width for the Editor

Some may want a fixed width for their tabs on the editor, so I added the feature.  This works for any tab position, but it's particularly useful for left/right positioned tabs, as currently geany doesn't handle large file names.

It was suggested in an email thread to use small/medium/large instead of a pixel width, but I think this is more versatile to allow the user any pixel width they want IMHO.  This would be like saying font sizes can only be small/medium/large, which would annoy some users that think small is too small but medium isn't small enough, etc.