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:

{F6331097}

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

{F6331095}

Reviewers: #kate, dhaumann

Reviewed By: #kate, dhaumann

Subscribers: kwrite-devel

Tags: #kate

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