Simplify/Speedup reading Stylesheet file

Summary:
read all of the information at once instead of concatenating
a QString over and over untill the end of the file. This removes
uneeded memory allocation and speeds up loading complex css files

Reviewers: hindenburg

Reviewed By: hindenburg

Subscribers: konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D17329