Avoid flickering during theme searches

[nacin]

• Searches are so fast, that they are tough to notice, especially if you are a touch typer. If the current theme is returned, it is even worse. In 3.7, the current theme was never shown below the fold, so this wasn't a problem.

• When a search matches an author name, we show the author in a clever overlay box. But, this is open to false positives. We should wait for the third letter to then show the author name. (We can still match the theme until then, just avoid flickering the author name.) Or we could debounce it, waiting for them to stop typing long enough before showing shorter matches. (To make Zé happy.)

[ehg]

debounce search on keyup and input

[nacin]

theme-search-debounce.diff​ seems like it could work. We should probably still wait until at least the second matched character to show authors, though.

[matveb]

Debounce looks good.

[nacin]

After some testing, let's avoid the debounce for now. There may be better way to produce feedback that a search occurred; this isn't it.

[nacin]

In 26686:

Themes: Only show author names during a search if it matches 3 or more characters.

props matveb.
fixes #26422.