Improve the UI of top bar on Mist

[alexvandesande]

Currently the mist UI uses a default native top bar that interferes with the current proposed design and it looks out of place.

[alexvandesande]

This is being worked on in the PR: #450

It has the current issues:

Close, minimize, maximize (zoom/fullscreen on mac) buttons are non-functional. From research it seems that while QML supports Qt.quit() and Qt.showMinimized() commands, those must be enabled by the lower level window object.

Resize windows is choppy and often causes the main window to lose it’s shadow effect. This might need some lower level access to the window object. The current resize method simply changes the x, y, width and height of the root qml object, there might be more optimised ways to do it.

In QML there is no way to find out the platform in order to change elements. This is needed because the position of the close/minimize/maximize buttons need to change according to the platform. Currently all buttons are left aligned, like on the mac, but this should be right aligned on windows and most linux distress (ubuntu being an exception).