PATCH: Support for HiDPI Displays on OS X (pgAdmin4)

Hi!

This quite trivial patch adds support for HiDPI (i.e. "retina") displays on OS X.

The reason for this issue is that you guys use a heavily trimmed down Info.plist file, which among others doesn't contain the NSPrincipalClass key.
Without that key an OS X application is treated as a legacy one and thus won't support many "new" features including HiDPI displays.
By adding the key back, you a) follow the recommended Info.plist style and b) get HiDPI support for free!

Apart from that I _really_ recommend adding the following keys too (mostly keys added by default by Xcode - I've guessed the values):

<key>CFBundleInfoDictionaryVersion</key>
<string>6.0</string>
<key>CFBundleName</key>
<string>pgAdmin 4</string>
<key>CFBundleShortVersionString</key>
<string>1.0.0</string>
<key>CFBundleVersion</key>
<string>1.0.0</string>
<key>LSMinimumSystemVersion</key>
<string>10.9</string>
<key>NSHumanReadableCopyright</key>
<string>Copyright © 2016 The pgAdmin Development Team. All rights reserved.</string>

Regards,
Leonard Hecker