Do not assume getSelectedPage() returns IEditorPart

Add check that selected page is actually an IEditorPart to avoid classcast exception.

I'm trying out emmet and noticed I hit this exception:

java.lang.ClassCastException: org.eclipse.swt.widgets.Composite cannot be cast to org.eclipse.ui.IEditorPart
	at io.emmet.eclipse.EclipseEmmetHelper.getTextEditor(EclipseEmmetHelper.java:53)
	at io.emmet.eclipse.TabKeyHandler.uninstall(TabKeyHandler.java:95)
	at io.emmet.eclipse.TabKeyHandler$2.partClosed(TabKeyHandler.java:216)
	at org.eclipse.ui.internal.WorkbenchPage$20.run(WorkbenchPage.java:4874)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
	at org.eclipse.ui.internal.WorkbenchPage.firePartClosed(WorkbenchPage.java:4872)
	at org.eclipse.ui.internal.e4.compatibility.CompatibilityPart$1.handleEvent(CompatibilityPart.java:101)
<stripped out eclipse even stackframes>

This only happens when opening JBoss Tools Properties page editor which does not return IEditorPart for getPages(). I'm investigating why that is but I would say it would be good if emmet would not assume this.

Here is a simple fix for this issue.