[applets/systemtray] Lazy initialize SystemTray model

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.