KateModeMenuList: move to QListView

Summary:
The new Mode menu introduced in D20958 is updated, removing the use of QListWidget.

This replaces:
QListWidget with QListView + QStandardItemModel
and KListWidgetSearchLine with QLineEdit

In addition, some changes are made:

• Decrease delay in displaying search results, after typing. The search clearing is done without delays.
• Add ToolTip in the search bar, with a brief description.

I also made small improvements to the code. From the user's point of view, this patch is almost imperceptible.

Reviewers: KTextEditor, cullmann, dhaumann

Reviewed By: KTextEditor, cullmann

Subscribers: ngraham, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D21712