Added optional transparency/blur to menu frames

Summary:
Added option to set transparency and blur behind menu frames such as right click context menu, toolbar menu, etc.
Transparency and blur is disabled by default.

Test Plan: {F5680253}

Reviewers: hpereiradacosta, #plasma, colomar, alake

Reviewed By: hpereiradacosta, #plasma

Subscribers: fredrik, alake, januz, abetts, colomar, andreask, zzag, ngraham, plasma-devel

Tags: #plasma, #breeze

Differential Revision: https://phabricator.kde.org/D10170