Skip tests related to RTMP plugin for Windows

The RTMP plugin is not included in GStreamer for Win32, so the test based on it will fail on Windows.
The workaround is to skip the RTMP test on Windows.
Updates to http errors, and validation

Changed HTTPError class to have more standard error messages, and
custom inner descriptions.

Decoupled JSON Schemas from validate module, now schemas are either
generated by config classes, or stored statically in the api blueprint.

Added JSON validation schemas to recording api Blueprint

Updated validation tests for validate.py changes, changed xunit setups
to pytest fixtures
Fixes #576 High/Low DPI Adapting

The Qt UI would change the size according to the logical DPI of the computer.
Only Windows and Linux have been tested. (Should also work on Mac but have not tested).
The unit test for the new file is also added.