Made double-click miniaturize respect window settings

Previously, if the window was set to not allow miniaturizing,
double-clicking on the title would still work and miniaturize the
window.

Using performMiniaturize: instead of miniaturize: seems to fix this and
not allow miniturization when the window does not support it.
Merge pull request #123 from Kapeli/master

Made double-click miniaturize respect window settings