re-work -debug switch handling

- re-work -debug help message text
- make -debug log every debugging information again (even all categories)
- remove unneeded fDebug checks in front of LogPrint()/qDebug(), as that
  check is done in LogPrintf() when category is != NULL (true for all
  LogPrint() calls
- remove fDebug ONLY in code which is NOT performance-critical
- harmonize addrman category name
- deprecate -debugnet usage, should be used via -debug=net and remove the
  corresponding global
transactionview: add message() signal

- allow to use message() in transactionview by connecting to the
  message() signal in WalletView
transactionview: make exportClicked() use message()

- use message() for displaying success or failure of export
- rework the strings to be more detailed / informative
- additional small cleanups
[Qt] sendcoinsdialog: convert QMessageBox usage to message()

- add new processSendCoinsReturn() function, which parses the
  status of WalletModel::SendCoinsReturn and generates a pair consisting
  of a message and message flags
- result is we only need one emit message() call
- this change ensures that the GUI is shown for warnings/errors in
  sendcoinsdialog, because of message() taking care of that for modal
  messages
- changes 2 warning message into error messages and removed "Error:"
  in front of the actual error message string
Merge pull request #3067 from Diapolo/debug-switch

re-work -debug switch handling
Merge pull request #3189

71ba467 [Qt] sendcoinsdialog: convert QMessageBox usage to message() (Philip Kaufmann)
Merge pull request #3159

9eb4ab6 transactionview: make exportClicked() use message() (Philip Kaufmann)
868d3ee transactionview: add message() signal (Philip Kaufmann)