Outline: Sorted function list

Summary:
-> As suggested by Kevin, wrapped the existing QuickOpenModel with the QSortFilterProxyModel to produce a sorted list of functions in the Outline drop down menu.
-> Converted the indexes from the Proxy Model to the Source Model and vice-versa where ever necessary.
-> Analyzed the use cases of the Outline menu, and modified all it's functionalities and features corresponding to the Proxy Model.

Test Plan:
-> Tested the working of the Outline and QuickOpen menu on a large project base.
-> Tested for the smooth functioning of widget expansion, both complete and partial.
-> Tested for Double Click, Single Click, Key Press, and other events.

BUG: 319404
FIXED-IN: 5.2.0

Reviewers: brauch, KDevelop, nellex

Reviewed By: brauch, KDevelop

Subscribers: kdevelop-devel, brauch, KDevelop

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D3673