Clean up CMerkleTx::SetMerkleBranch.

The case SetMerkleBranch(NULL) was never actually used, and thus the
involved code (loading the block from disk) can be removed and the
implementation simplified.
[Qt] include and file header cleanup

- alphabetical ordering
- correct ordering own headers before normal headers etc.
Fixing condition 'sabotaging' MSVC build
Merge pull request #4947

6134b43 Fixing condition 'sabotaging' MSVC build (ENikS)
Merge pull request #4852

5e83bc4 [Qt] include and file header cleanup (Philip Kaufmann)
Merge pull request #4787

4b0deb3 Clean up CMerkleTx::SetMerkleBranch. (Daniel Kraft)