fix misplacement of line number when window body width < total width

[guiyuanju]

When using some mode like Olivetti-mode which changes the window-body-width to center the text, the original calculation method will place the line / column number near the middle of mode-line, I don't know if it is on purpose, but I saw the commented code that use window-total-width which forgot the window-right-divider-width, so I made this pull request.

Note the right-up corner, the line column number place: Before:

After:

[rougier]

Problem is when we have two windows side to side and large divider (such as the default), it also make computation a bit wrong. Maybe it would be best to have a function that return the window width according to current mode or some other conditions. Is there a simple test for testing if in Olivetti mode?

[guiyuanju]

Sorry I'm not sure if it is what you mean by "have two windows side to side and large divider", but I've tested with two windows open and the window-divider-default-right-width set to a bigger number like 256 pixels, and reenable the window-divider-mode to activate the new divider, it seems no problem both in normal mode and Olivetti mode .

[rougier]

Ok, I've to test it as bit more then.

[rougier]

Did you test with your proposed patch ?

[guiyuanju]

yes with my patch

[rougier]

Ok, great. Can you rebase your pull request (I did a lot of changes) ?