Qt: new icons

- flat and b/w icons seems to be the new trend
- support for HiDPI/retina
- change icon sizes
- fix assets-attribution.md
- removed unused icons
[Qt] the RPC Console should be a QWidget to make window more independent

- fix issue #5254
Add /rest/headers
minor fix on script test data docs
Merge pull request #5482

87e8e1c minor fix on script test data docs (Manuel Araoz)
Merge pull request #5444

f676c80 Add /rest/headers (Pieter Wuille)
Merge pull request #5219

31aac02 Qt: new icons (Jonas Schnelli)
[RPC-TESTS] ability to run single test manually

adds possibility to run ./qa/pull-tester/rpc-tests.sh <TESTNAME>
Qt: HiDPI (retina) support for splash screen

- remove splash screen images (reduce binary size)
- dynamicly draw splash screen with available icon.
- remove testnet icon
- dynamicly colorize testnet icon
Qt: Go back to using QIcon functionality for scaling
resize tray icon because a 1024x1024 icon won't show in ubuntu (bug)
Merge pull request #5228

ac23394 resize tray icon because a 1024x1024 icon won't show in ubuntu (bug) (Jonas Schnelli)
8e76ca0 Qt: Go back to using QIcon functionality for scaling (Wladimir J. van der Laan)
54f2571 Qt: HiDPI (retina) support for splash screen (Jonas Schnelli)
Merge pull request #5453

1340f00 [RPC-TESTS] ability to run single test manually (Jonas Schnelli)
Fix small typos in comments and error messages

Rebased-From: 67b2d819cdf6181e7f016e5366ce7479830893bd
Github-Pull: #5404
Merge pull request #5258

4a8fc15 [Qt] the RPC Console should be a QWidget to make window more independent (Jonas Schnelli)
[Qt] fix a <Qt4.7 compatibility issue raised in #5228
Merge pull request #5487

270f42d [Qt] fix a <Qt4.7 compatibility issue raised in #5228 (Jonas Schnelli)
[Qt] light blue icon color for regtest

Changing the icons color for regtest in now lightweight.
Merge pull request #5488

ff63801 [Qt] light blue icon color for regtest (Jonas Schnelli)
Create developer-notes.md

Moves coding guidelines and development tips/tricks into a single file.
Also adds a section explaining pull request terminology.