Indentation API is inconsistent

[LeonMatthesKDAB]

Currently, we have these indentation functions on TextDocument:

1. indent (int count)
2. string indentationAtPosition(int pos)
3. removeIndent(int count)

The 1. and 3. are consistent, but indentationAtPosition should ideally also return an int, as then you can do things like:
"Indent this line 1 more than this other line".
That would probably require us to add a "indent-spaces" property or some such for each document, that defines how many spaces count as one indentation (or whether it's tab?)

Ideally, an API for working line-based would also be useful.
e.g.:

1. indentLine(int line, int count)
2. int indentationAtLine(int line)