add DEFAULT_TRANSACTION_FEE constant in wallet

- as this is a shared Core/GUI setting, this makes it easier to keep them
  in sync (also no new includes are needed)
[Qt] fix Qt slot problem in receivecoinsdialog

- fixes error from debug.log:
  QMetaObject::connectSlotsByName: No matching signal for
  on_recentRequestsView_selectionChanged(QItemSelection,QItemSelection)
- small style fixes (e.g. alphabetical ordering if includes etc.)
- fixes #3992
Typo Fix In decoderawtransaction Help: s/txid/hex/

Help text appears to have been copy/pasted from getrawtransaction,
so it erroneously asked for a txid where rawtransaction hex should appear.

Remove lines which were copy/pasted from getrawtransaction but which
aren't displayed by decoderawtransaction.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 5cc0133 80c521e
Github-Pull: #4106
qt: fix compile issue in Qt GUI

This was introduced in 3e1cf9b. Needs a cast to qint64.
build: fix: remove error output

while git not found in path, error is output to console.