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 whether the control content is empty
	 */
	private boolean isControlContentEmpty() {
		return getControlContentAdapter().getControlContents(getControl())
				.length() == 0;
	}
	
	/*
	 * The popup has just opened, but listeners have not yet
	 * been notified.  Perform any cleanup that is needed.
	 */
	private void internalPopupOpened() {
		
		if (control instanceof Combo) {
			((Combo)control).setListVisible(false);
		}
	}
	
	/*
	 * Return whether a proposal popup should remain open.
	 * If it was autoactivated by specific characters, and 
	 * none of those characters remain, then it should not remain
	 * open.  This method should not be used to determine
	 * whether autoactivation has occurred or should occur, only whether
	 * the circumstances would dictate that a popup remain open.
	 */
	private boolean shouldPopupRemainOpen() {
		
		if (autoActivateString == null || autoActivateString.length() == 0)
			return true;