Add force save behavior to KEntryMap

Summary:
Fix the following bug, if an entry is set on HOME/.config/kdeglobals and on a specific config file like kcmfonts
When you hit restore defaults button and apply, value will be not wrote on the specific file, but then the value is the one from kdeglobals
This patch ensure we write the key to the specific configuration file on those case with an empty value. KConfig will take default value automatically

Test Plan:
Added a test to ensure flag is working
Tested using some KCM to ensure all is working fine

Reviewers: ervin, dfaure, meven, crossi, hchain

Reviewed By: ervin, dfaure, meven

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28128