Change the default shortcut of 'Clear Scrollback and Reset' to Ctrl+Shift+K

The current default shortcut is Ctrl+Shift+X, which might be accidentally
pressed when users intend to press Ctrl+Shift+C for copying or Ctrl+Shift+V for
pasting. Scrollback is important asset, but it is impossible to recover
once gone. So change the default shortcut to decrease the chance of
accidents.

FEATURE: 282593
FIXED-IN: 4.9
REVIEW: 103660

src/SessionController.cpp

@@ -517,7 +517,7 @@ void SessionController::setupActions()
     action = collection->addAction("clear-history-and-reset", this, SLOT(clearHistoryAndReset()));
     action->setText(i18n("Clear Scrollback and Reset"));
     action->setIcon(KIcon("edit-clear-history"));
-    action->setShortcut(QKeySequence(Qt::CTRL + Qt::SHIFT + Qt::Key_X));
+    action->setShortcut(QKeySequence(Qt::CTRL + Qt::SHIFT + Qt::Key_K));
 
     // Profile Options
     action = collection->addAction("edit-current-profile", this, SLOT(editCurrentProfile()));