Reduce flickering in palette

[MinusGix]

Lapce Version

HEAD

System information

Linux (Wayland)

Describe the bug

When typing quickly in palette, especially in something that has previews like #blah then it flickers.
This is probably due to removing the existing entries and then loading in the new entry, but since it isn't ready immediately it will flicker.
I think there's two places that are doing this.

• the palette options
• the preview editor

I think these both can be fixed by delaying clearing until the new values are loaded in. Possibly locking any actions from occurring while we wait for it to load in. (And of course removing any lock once it loads in, or it stops such as due to closing the palette)