Select-all [Palette Search] text upon utilizing shortcut command for fresh searching

After inquiring as to whether clearing palette search upon [palette search command] invocation seemed appropriate to others, the response lent toward adapting the default application behavior of selecting/highlighting search-text. This would prepare for new search entry, and if by the off-chance the user wanted to continue previous searching key, a right arrow would serve the purpose. It doesn't seem to be normal/default behavior to do this upon clicking.

Actually, web browsers tend to highlight the URL fields when clicking in them, but many applications don't do this when mouse-clicking into a search text field, so I left that alone, for now ;)

The conversation was/is @ https://musescore.org/en/node/300485

Anyways. Will do a simple little pull request for this, as I find it convenient for my machine and figured some others might find this appropriate also.