This PR improves line decoration handling:

1. Double clicking on an edge invokes Line Decoration dialog. This aims to improve productivity when creating a drawing. The changes involved are in files QGIEdge.h and QGIEdge.cpp .
2. When the Line Decoration dialog is invoked, all updates to style are immediately reflected. This is perfect. However when Cancel is pressed, the old style values were not stored anywhere and in fact, there was no difference between pressing OK or Cancel. It just closed the dialog. With this PR, the line styles are fully preserved and restored, if the user decides to press Cancel. These changes are contained in TaskLineDecor.h and taskLineDecor.cpp files.
3. Small fix of earlier refactorization. Without setting the pen style, all lines are drawn as solid. Code change in QGIPrimPath:311.

I hope you will find this addition useful, in case of any issues or questions, please leave a comment!