Introduce option to disable relay/mining of bare multisig scripts in TX outputs

First and foremost, this defaults to OFF.

This option lets a node consider such transactions non-standard,
meaning they will not be relayed or mined by default, but other miners
are free to mine these as usual.
qt: Make error message for failed export a little friendlier

Closes #4528.
Merge pull request #3939

3da434a Introduce option to disable relay/mining of bare multisig scripts in TX outputs (Jeff Garzik)
[Qt] Fix thin space in URI
Merge pull request #4563

027dcdc [Qt] Fix thin space in URI (Cozz Lovan)
qt: Move SplashFinished to after ClientModel/WalletModel creation

With a large wallet there was a noticable gap between hiding of the
splash and showing the main window.
Revert "Remove signal DoubleSpendDetected, use function"

This reverts commit 0da6b3fd187da3aa810aaa584d8bd197ad4fa2b9.
Revert "Check signatures before respend relay"

This reverts commit 88dd3598d22197a22565e524cecdc08107cf76ac.
Revert "Formatting, spelling, comment fixes."

This reverts commit 7a19efe04069d9a1e251cdc94b25184f76d9d901.
Revert "Add release notes entry"

This reverts commit 9fa53dd3bdc6f62b16a7c2b970449c8c35f4c41b.
Revert "Add -respendnotify option and new RPC data"

This reverts commit 9004798e62e987ddf50030b17fa1881b63dd5e45.
Revert "UI to alert of respend attempt affecting wallet."

This reverts commit ada5a067c75f19a724cc054286ecf2254e5dbe8f.

Conflicts:
	src/qt/guiconstants.h
	src/wallet.h
Revert "Relay double-spends, subject to anti-DOS"

This reverts commit d640a3ceab4f4372c2a0f738c1286cfde4b41b50.
Revert "CBloomFilter::clear() method"

This reverts commit 8fbf03995df9a2003be603be1a930bc3373d56e0.