Implement SI-style (thin space) thoudands separator

Personally, I find long strings of digits (either side of the decimal point) hard to read....  To the right of the decimal point, it's OK as long as you really only care as to the first few..... But when you have multiple leading zeros 0.001 and 0.0001 are easy to confuse (but no big loss)

But switching to milli- or micro- and 1000000 versus 100000 is a big deal.

IMHO the only sane option is SI-style (i.e. a thin space as thousands separator, both left and right of the decimal point) because:

* given the huge variation in punctuation marks used as decimal separators and thousands separators, anything else is confusing at best and ambiguous at worst
* there's really no other widely used convention for thousands separators to the right of the decimal point

This pull request adds thin spaces to formatted BTC amounts (both sides of the decimal point), *but*

Thin spaces are avoided in the following cases:

* Copying from the transaction view (I and I'm sure countless others rely on being able to cut and paste BTC values into spreadsheets)
* Transaction exports (for similar reasons)

(BTW, i never even looked at Qt before yesterday and this ended up involving somewhat more non-trivial Qt code than I anticipated- so this really should be reviewed by someone who knows Qt)

ETA: Are there other cases where we should avoid thousands separators?  (Or alternatively should the code avoid them by default and only insert them in certain cases - and if so in which cases?)