Merge #14374: qt: Add "Blocksdir" to Debug window

Summary:
PR14374 backport https://github.com/bitcoin/bitcoin/pull/14374/files
2ab9140c92c7ffd950f9ea6e1e78107a217bb336 Add tooltips for both datadir and blocksdir (Hennadii Stepanov)
3045704502e8a241b60b847fd52fcbed3129a2e4 Add "Blocksdir" to Debug window (Hennadii Stepanov)

Pull request description:

To get the current `blocksdir` is valuable for debug purposes after
merging #12653.

![screenshot from 2018-10-02 23-16-52](https://user-images.githubusercontent.com/32963518/46374770-2ef6f580-c69a-11e8-85c2-44a49fa36b28.png)

Backporter note: the displayed "Blocksdir" is not the same as the provided
command line -blocksdir parameter:

no -blocksdir and no -datadir:
Blocksdir /home/mark/.bitcoin/regtest/blocks

with -blocksdir=/home/mark/blocksdir:
Blocksdir /home/mark/blocksdir/regtest/blocks

Test Plan:
make check
Observe behaviour noted above.
Observe tooltip.

Reviewers: #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3158