Ignore ongoing text drag selection when user writes to textbox

[frenzibyte]

• Closes Fast typing makes issues when changing sample volume osu#28713

Before:

CleanShot.2024-07-10.at.12.29.07.mp4

After:

CleanShot.2024-07-10.at.12.28.38.mp4

[bdach]

Looking at other textboxes on windows, I notice that for most textboxes, when dragging, pressing keys will overwrite the current selection and continue typing, until the mouse is moved, at which point drag will select again. Current master also does that.

This changes that behaviour, but for instance, I see VS Code doing the same thing as this does, in that a drag operation gets killed by keyboard input entirely and a new one has to be started to select again.

Code looks correct, and I'm mostly indifferent on the UX, so leaving it to other reviewers to determine if we want this in this form. Maybe this varies per-platform too, not sure.