[mediaframe applet] Fix broken selection of fill mode

Summary:
"comboBoxItems" does not reference anything, at least after
2700c2f7c5d52aff276bea3774b5bd3552d91157
Instead the model of the combobox created by JS due to i18n() calls needs
to be indirectly referenced.

To not only avoid some race condition during setup, text of the
fillModeDescription label is now set by a binding, instead of being
manually set in onCurrentIndexChanged handler of the combobox

Test Plan:
Description of fill mode is now correct on initial config page view.
Selecting another fill mode results in update of the description.
Selecting another fill mode now also actually works.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12134