For a long time, I've had my keyboard configured to use the layout currently known as "English (international AltGr dead keys)." I like this layout because without any modifier keys, it's identical to the US English keyboard, but when I hold Right Alt I can get accented letters and other characters not available on a standard US English keyboard.
In Oneiric, however, the layout is messed up. Right Alt+N produces "ñ" as expected. And another method works: Right Alt+`, E produces "è", also as expected. But there's no way to type "é", which is probably the accented letter I type the most. I expect Right Alt+A, E to do the trick. But instead of a dead key for the acute accent, it uses a method for combining characters to create the hybrid "́e". This hybrid looks like the proper "é" in some settings, but it isn't the same character and doesn't always work. (For example, in the text input box as I type this, it looks the same as the proper character, but when displayed on the site for all so see, it looks very wrong–at least on my machine.) Ditto for all other characters with an acute accent, though some are available directly as pre-composed characters: For example, Right Alt+I yields "í".
How can I change the acute accent on the A key to a proper dead key? Perhaps the more general version of this is: How can I tweak my keyboard layout?
Update
I just tested this on my other machine, also running Oneiric, but upgraded from previous versions. I have no problems with the second machine. The problem machine was a fresh install of Oneiric, but I kept my old $HOME when I did the fresh install.
Clarification
Even if an answer doesn't address my specific examples, I would still accept it if it provided enough detail for me to find the layout and tweak it according to my needs.
Major Update
After working through the information gained through Jim C's and Chascon's helpful replies, I've learned something new: The problem isn't with the layout itself, but with the fact that the selected layout isn't being applied. When I look at the definition in /usr/share/X11/xkb/symbols/us of the layout I've been running for a long time, I found that the definition doesn't match what I get when I type. In addition, the keyboard layout dialog that's supposed to show the current layout looks different from the way the layout is defined in the file I mentioned, and matches what actually happens when I type.
Following Jim C's suggestion, I created a new layout in /usr/share/X11/xkb/symbols/us containing some modifications to the layout I want. I can select my layout from the keyboard properties, and I can use in on the console following Chascon's post, but the layout I get when typing is unchanged.
Apparently, there's a different layout defined somewhere that's overriding what I've set. Where is that layout hiding?
This problem occurs in Unity (3D and 2D), but I was able to get the correct layout set in Xfce.
In case it's relevant, this problem has occurred since I installed Oneiric fresh on this machine (though I preserved my $HOME). I don't recall whether this problem occurred before the reinstall.
Also, in case it's relevant, I also run iBus so I can type Korean. I have a few difficulties with iBus, but I doubt they're related.
Best Answer
I'll tell you what eventually worked best for me.
The previous two steps have been suggested as fixes on their own (on the Ubuntu forums?). They did not work for me. I tried them together consecutively and this again did not work for me. Thus my next step.
I think my problem was from the fact that some of the layouts had changed in name, at least that's my take considering that the layout names weren't exactly what I expected when reassigning them. Deleting my extra layouts and then assigning new ones from a fresh list seems to have circumvented my issues, that being that the layouts of my choice now point to existing layouts with names to match.