Allow to use native titlebar on dialogs

As the title says, I like to be able to use native titlebars on juce dialogs (like the DeviceManager).
I made this for myself, and decided to share in case someone sees some use on it

Patch against latest git here:
http://distrho.git.sourceforge.net/git/gitweb.cgi?p=distrho/distrho;a=commitdiff;h=5174725aeb2af0bb48a0a2396334c77ad5a4b1d7#patch3