Allow Palettes search box to respond to mouse clicks and swipes (and Delete key)

OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 3.5.2.311459983, revision: 465e7b6

If you enter a search term in the Palettes search box, you cannot select it by double-clicking, nor by swiping it (thus the [Delete] key is inoperative). IMV, users would expect to be able to use these commands.