Summary:
SystemTray model was created in constructor, delay initialization and
create it lazily.
The `SystemTray::appletAdded` signal is connected to both SystemTray's
model and `Container`. `Container` must be connected first, because it sets
some properties of newly added applet: `visible` to false and `parent`
to the root item (container). If `Container` is connected second, then
newly created applet ios added to the model and rendered, but immediately
after that `Container` changes visibility to false.

BUG: 420993

Test Plan:
1. Do not play anything (no Medial Player icon at all)
2. Run Gwenview
3. Check if Medial Player icon in hidden view is rendered correct.
Before change:
Sometimes icon is not rendered, looks like invisible or fully transparent.
After change:
Icon rendered correctly.