Core: make ICore interface static

This mainly serves two purposes:
(a) it saves a function call in ICore::instance()->foo() vs ICore::foo() at runtime
(b) it saves typing and reduces line noise when reading

It's also (mostly) source compatible, as ICore::instance()->foo() remains compilable.

Change-Id: Icf7be0bce17fefe3560473534a8991ff79cbecc3
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

src/plugins/coreplugin/coreplugin.pro

@@ -57,7 +57,6 @@ SOURCES += mainwindow.cpp \
     coreplugin.cpp \
     variablemanager.cpp \
     modemanager.cpp \
-    coreimpl.cpp \
     basefilewizard.cpp \
     generatedfile.cpp \
     plugindialog.cpp \
@@ -160,7 +159,6 @@ HEADERS += mainwindow.h \
     coreplugin.h \
     variablemanager.h \
     modemanager.h \
-    coreimpl.h \
     basefilewizard.h \
     generatedfile.h \
     plugindialog.h \