Use correct arrow cursor for Annotations' close buttons

Summary:
When hovering the mouse over the X symbol to close a note the cursor remains the same of the current shape (hand, selection cursor) based on the tool selected.
This patch morphe the cursor to the standard arrow over the X symbol to close a note by adding setCursor() call in the CloseButton constructor

 BUG: 384381

Test Plan:
1.  Open okular and select Tools -> Review or press F6 to create a new pop-up note
2. Hover the mouse cursor over the close button (X) of the pop-up note window
3. You will see the standard arrow instead of the current tool based cursor.

Screenshot below (the tooltip saying "Close this note" was not captured by Spectacle application)
{F5685539}

Reviewers: #okular, ngraham

Reviewed By: ngraham

Subscribers: ngraham, #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D10209