Substring search when selecting kits should allow time for several keystrokes before starting

To reproduce:

Start Qt Creator. Ideally, have several dozen kits installed. (multiple versions/toolchains + MCU versions/boards, etc.)

Pick a project from Welcome screen

See kit selection window, start typing in the search box.

Observe that QtC kicks off a full substring search instantaneously with each keystroke, and that this search blocks the appearance of subsequent keystrokes (each of which kicks off its own search with blocking delay).

Suggestion: delay for e.g. 500ms before kicking off the search, to allow time for several keystrokes to accumulate.

This fix may be generalizable to other instances within the application. Apply there as well, if so.