QuickOpen: Trim whitespace from input

Summary: Trim whitespace from the input to QuickOpen. For example, when copying a file path into the QuickOpen widget from some command line output and it happens to contain some whitespace at the start or end, it will still work with this patch. Without the patch, one has to remove the whitespace manually.

Test Plan: Shortly tested it manually.

Reviewers: #kdevelop, dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann, apol, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D17885