Show view mode buttons in the open/save dialog's toolbar

Summary:
Without this patch, changing the view mode is unnecessarily difficult: you need to click on the settings button in the corner, then navigate to a sub-menu if you want to change the view mode.

This patch adds toggle buttons for the two most useful view modes (Short view and Detailed view) to the toolbar.

Test Plan:
Compile and deploy patch, create and log into a new user account, open Kate, open a file open or save dialog, and see that there are buttons to switch between Short View and Detailed Tree View:

Reviewers: Frameworks, rkflx, VDG, abetts

Reviewed By: rkflx, VDG, abetts

Subscribers: sharvey, rkflx, mmustac, broulik

Tags: Frameworks

Maniphest Tasks: T8552

Differential Revision: https://phabricator.kde.org/D12077