Bug 250604 [FieldAssist] API - ContentProposalAdapter needs isShowing() method
Please implement a method ContentProposalAdapter.isShwoing(). The method should return a boolean whether the content assistant proposal popup is open or not.
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ContentProposalAdapter.java
* 
	 * @return <code>true</code> if the adapter is enabled, and
	 *         <code>false</code> if it is not.
	 */
	public boolean isEnabled() {
		return isEnabled;
	}
	/**
	 * Set the label provider that is used to show proposals. The lifecycle of
	 * the specified label provider is not managed by this adapter. Clients must
	 * dispose the label provider when it is no longer needed.
	 * 
	 * @param labelProvider
	 *            the (@link ILabelProvider} used to show proposals.
	 */
	public void setLabelProvider(ILabelProvider labelProvider) {
		this.labelProvider = labelProvider;
	}
	/**
	 * Return the proposal provider that provides content proposals given the
	 * current content of the field. A value of <code>null</code> indicates
	 * that there are no content proposals available for the field.
	 * 
	 * @return the {@link IContentProposalProvider} used to show proposals. May
	 *         be <code>null</code>.
	 */
	public IContentProposalProvider getContentProposalProvider() {
		return proposalProvider;
	}
	/**