Rename DisplayObjectContainer#addChildAtDepth to addTimelineObjectAtDepth

The method behaves very differently from `addChildAt`, so the name shouldn't be this close.
Rename DisplayObjectContainer#getChildAtDepth to getTimelineObjectAtDepth

Same as the previous patch, but for the associated getter.
Cleanup in displayObjectContainer.js unit tests file
Add test for add-, get-, and removeChildAt to displayObjectContainer.js
Add test for getChildByName to displayObjectContainer.js
Remove flash_display_DisplayObjectContainer tracetest

This is now fully covered by unit tests
Remove flash_display_Graphics tracetest

This test never really tested anything except for some methods not throwing.
Remove flash_system_System tracetest

We don't really implement the functionality this tests, and it's *so* very much low priority to do so, that this test just isn't worth keeping.
Remove security-related tracetests

These only test that some getters exist and don't return falsy values. Not worth keeping.
Remove flash_system_Capabilities tracetest

Only tests that some getters exist and don't return falsy values. Not worth keeping.
Remove unused flash_system_ApplicationDomain test files
Remove flash_display_Shape tracetest

This tests as close to nothing as seems possible.
Remove filter-related tracetests

While this technically do provide some coverage, it's so basic that keeping them around doesn't give us any real upsides.

We should work on implementing proper unit and ref tests for filters, though.
Remove flash_ui_ContextMenu tracetest

While this technically does provide some coverage, it's so basic that keeping it around doesn't give us any real upsides.