Add navigation history to forward/back buttons

Summary:
Adds navigation history to forward/back buttons in the toolbar. This
changes the forward/back buttons in the toolbar to use the
KToolBarPopupAction class which provides access to a drop down menu.

Test Plan:
Browse some folders
Click the back drop down menu and navigate somewhere
Click the forward drop down menu and navigate somewhere

FEATURE: 157819
FIXED-IN: 19.12.0

Reviewers: #dolphin, ngraham, elvisangelaccio, #vdg

Reviewed By: #dolphin, ngraham, elvisangelaccio, #vdg

Subscribers: felixernst, nerdopolist, mart, richardl, ognarb, david.fontanals, abetts, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19311