Quick Open: Restrict file column size to take up at most half of the view

Summary:
This is how it looks like if the git project contains very long filenames:

After the change, it looks like this: the left column take at most half of the horizontal space:

Reviewers: Kate, dhaumann

Reviewed By: Kate, dhaumann

Subscribers: kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D16637