Add test for Inspector module, fix children order for ModalView

[KeyWeeUsr]

ref #5369

The PR still needs the last part of unittest i.e. testing multiple ModalViews or Popups opened and check if they stack properly as LIFO.

[KeyWeeUsr]

meh, the tests will crash because of Travis 😢

The log length has exceeded the limit of 4 MB (this usually means that the test suite is raising the same exception over and over).

The job has been terminated

Edit: Resolved with silencing KV lang specific TRACE type of logging in this particular TestCase. Basically tracing KV isn't necessary for this particular test as the asserting checks for values modified by the python side only i.e. if a widget is selected, if the on_* event works, etc.

[KeyWeeUsr]

Tests pass, the additional change in inspector.py closes #5369.