Fix ComboBox & mode description for image fill mode

Summary:
BUG: 390571

Reworked ConfigGeneral.qml to properly display ComboBox for image
fill modes, along with descriptive message explaining fill mode.

Changed default change interval (main.xml) to 10.0 instead of the
random-appearing 6.8 original.

(Minor) Capitalized "Frame" in plasmoid title as "Media Frame"

Reviewers: #plasma, mart, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10618