Improvements to EditorResourcePicker

[KoBeWi]

The PR does various improvements to EditorResourcePicker

• added support for Image -> ImageTexture conversion (part of Add more inspector Resource auto-conversions #55727)
• when doing conversion and a matching resource already exists, don't create new resource, but replace data instead (this is somewhat similar to Assign value to property by dropping to scene tree #50517, e.g. you can drop a texture on an existing StandardMaterial3D to replace current texture instead of creating new material)
• fixed a bug where clearing resource wouldn't clear the tooltip
• fixed Texture -> StandardMaterial3D not working
• added resource type to tooltip (similar to scene tree dock)
  

[YuriSizov]

Looks good at a glance! Note that some names have changed in master, so a cherrypick would require manual renames. Probably worth making a dedicated 3.x PR.

[akien-mga]

Thanks!

[timothyqiu]

Cherry-picked for 3.6