Pin items feature

[SergioRibera]

Description

This PR includes the functionality to pin and unpin existing elements in the clipboard history

Expected functionality

• when you click the pin button on a previously unpinned item, it is added to the list of pinned items and removed from the current history list
• the pinned items appear at the top of the list showing from the most recent to the oldest one
• when you click on an already pinned item, it is removed from the pinned list and dies there (it is not added to the normal clipboard list)
• when you left click on a pinned item it is treated as a new item in the clipboard so it is added normally to the clipboard list and kept in the pinned list
• when you clear the clipboard the pinned items do not disappear, the only way to remove a pinned item is to click on the pin button in "unpin" mode

Before to make PR

• [x] run cargo make without errors
• [x] update Readme
• [x] correctly fulfills the expected functionality