Change the open/close file tree icons

Created by: AlicjaSuska

Part of the tracking issue #16165, RFC 250

Description

Change the icons used for opening and closing the file tree in order to make them consistent with the action items bar.

Design details

• Figma
• Close: change the icon from 'X' to arrows mdi:chevron-double-left
• Open: change the icon to mdi:file-tree, change the background, make the background and the clickable area bigger
• Add tooltips (delay 1s)