Increase requirement for building Qt Creator from Qt 5.6.2 to Qt 5.9

Qt 5.11 will be released and Qt 5.9 is the new LTS.
Increasing the requirement allows us to remove some baggage and
workarounds like Utils::asConst, special Q_OVERRIDE header, and
deployment target adaptions on macOS, and simplifies pulling in
proparser changes from Qt.

Change-Id: I9faf99336d26dd20bd63e6904ae543c0f351aa4d
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

README.md

@@ -14,7 +14,7 @@ The standalone binary packages support the following platforms:
 
 Prerequisites:
 
-* Qt 5.6.2 or later
+* Qt 5.9.0 or later
 * Qt WebEngine module for QtWebEngine based help viewer
 * On Windows:
     * ActiveState Active Perl
@@ -66,7 +66,7 @@ For detailed information on the supported compilers, see
        for example, `c:\work`. If you plan to use MinGW and Microsoft Visual
        Studio simultaneously or mix different Qt versions, we recommend
        creating a directory structure which reflects that. For example:
-       `C:\work\qt5.6.0-vs12, C:\work\qt5.6.0-mingw`.
+       `C:\work\qt5.9.0-vs12, C:\work\qt5.9.0-mingw`.
 
    4.  Download and install Perl from <https://www.activestate.com/activeperl>
        and check that perl.exe is added to the path. Run `perl -v` to verify
@@ -476,4 +476,3 @@ SQLite (https://www.sqlite.org) is in the Public Domain.
   This Font Software is licensed under the SIL Open Font License, Version 1.1.
 
   The font and license files can be found in QtCreator/src/libs/3rdparty/fonts.
-

qbs/imports/QtcProduct.qbs

@@ -20,7 +20,7 @@ Product {
     Depends { name: "cpp" }
     Depends { name: "qtc" }
     Depends { name: product.name + " dev headers"; required: false }
-    Depends { name: "Qt.core"; versionAtLeast: "5.6.2" }
+    Depends { name: "Qt.core"; versionAtLeast: "5.9.0" }
 
     // TODO: Should fall back to what came from Qt.core for Qt < 5.7, but we cannot express that
     //       atm. Conditionally pulling in a module that sets the property is also not possible,

qtcreator.pro

@@ -1,9 +1,9 @@
 include(qtcreator.pri)
 
 #version check qt
-!minQtVersion(5, 6, 2) {
+!minQtVersion(5, 9, 0) {
     message("Cannot build $$IDE_DISPLAY_NAME with Qt version $${QT_VERSION}.")
-    error("Use at least Qt 5.6.2.")
+    error("Use at least Qt 5.9.0.")
 }
 
 include(doc/doc.pri)