Faster drag&drop in directories with thousands of files

Summary:
The check is called when the mouse is moved in a drag&drop operation.

Dragging all files in a directory with 3000 files under callgrind,
moving the mouse to the other panel and then canceling, doing it twice,
callgrind shows that the method urlListMatchesUrl is called around
200 times, spending around 9,30% of the cpu in those calls.
Applying the patch, callgrind tells it uses now 0.31% of the cpu in 1208 calls.

CCBUG: 342056

Reviewers: #dolphin, elvisangelaccio, markg

Reviewed By: #dolphin, elvisangelaccio, markg

Subscribers: markg, anthonyfieroni, michaelh, elvisangelaccio, ngraham

Differential Revision: https://phabricator.kde.org/D10085