Fix #628 Unload Input Source Problem

Now the user can load and unload video/audio input sources
and record without closing the program.

This bug was caused because the flag variables
self.record_audio and self.record_video are not cleared
after a cycle of recording ends. So the program will
try to unload the source even it has been unchecked.
This fix reset the variables by the end of recording
so it will not influence the next one.
Fix #638 Set "Record Region" after selecting region

- Automatically check the "Record Region" button after the user selects
  a region to record.
- Rename the "Set" button to "Select Region".
Related #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 function is unfinished.