qt: define QT_NO_KEYWORDS

QT_NO_KEYWORDS prevents Qt from defining the `foreach`, `signals`,
`slots` and `emit` macros.

Avoid overlap between Qt macros and boost - for example #undef hackiness
in #6421.

Conflicts:
	src/qt/addressbookpage.cpp
	src/qt/peertablemodel.cpp
	src/qt/receivecoinsdialog.cpp
	src/qt/rpcconsole.cpp

Rebased-From: d29ec6c2301e593d577126d1ca85b93307b32bf1
Github-Pull: #6433

src/qt/utilitydialog.cpp

@@ -84,7 +84,7 @@ HelpMessageDialog::HelpMessageDialog(QWidget *parent, bool about) :
         QTextCharFormat bold;
         bold.setFontWeight(QFont::Bold);
 
-        foreach (const QString &line, coreOptions.split("\n")) {
+        Q_FOREACH (const QString &line, coreOptions.split("\n")) {
             if (line.startsWith("  -"))
             {
                 cursor.currentTable()->appendRows(1);