Don't instantiate ColorPicker in EditorPropertyColor constructor

[larspet]

• Fixes Inspector loads slowly when there's many Color properties #101569

The EditorPropertyColor constructor was using ColorPickerButton::get_popup() and ColorPickerButton::get_picker(), which was instantiating the ColorPicker without it being opened first. This led to 40+ pickers being instantiated in some cases.

This PR avoids calling those getters by utilizing the ColorPickerButton::toggled signal instead.

On a debug build I get these results (quickly measured by eye & timer):

• Opening Editor Settings > Text Editor > Theme:
  • Master: 2 sec
  • This PR: instant
• Opening the MRP inspector:
  • Master: 1.5 sec
  • This PR: 0.2 sec (must be other Node properties making it not instant)

Switching away to another setting or inspector is faster now too, since there's no need to delete all the pickers.

However, now that it's using a different signal there may be a regression somewhere. The pickers work using both mouse or keyboard though.