Generate documentation .tags files for all of the docs

This allows others to link to QtDBus and the other libraries/tools using
doxygen.

Fixes: QTBUG-60933
Change-Id: I026895a432a328f224c40cf231ad12d109dc648f
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>

qmake/doc/qmake.qdocconf

@@ -25,6 +25,8 @@ sources                 = src/qmake-manual.qdoc
 imagedirs               = images
 exampledirs             = snippets
 
+tagfile = qmake.tags
+
 depends += \
     activeqt \
     qt3d \

src/dbus/doc/qtdbus.qdocconf

@@ -21,6 +21,8 @@ excludedirs += ../../../examples/widgets/doc
 
 examplesinstallpath     = dbus
 
+tagfile = qtdbus.tags
+
 depends += qtdoc qtcore
 
 # The following parameters are for creating a qhp file, the qhelpgenerator

src/opengl/doc/qtopengl.qdocconf

@@ -23,6 +23,8 @@ depends += qtdoc qtcore qtgui qtwidgets qmake
 
 examplesinstallpath     = opengl
 
+tagfile = qtopengl.tags
+
 # The following parameters are for creating a qhp file, the qhelpgenerator
 # program can convert the qhp file into a qch file which can be opened in
 # Qt Assistant and/or Qt Creator.

src/platformheaders/doc/qtplatformheaders.qdocconf

@@ -35,5 +35,7 @@ sourcedirs  += ..
 exampledirs += snippets
 imagedirs   += images
 
+tagfile = qtplatformheaders.tags
+
 navigation.landingpage = "Qt Platform Headers"
 navigation.cppclassespage = "Qt Platform Headers C++ Classes"