Remove styles overriding standard widgets

Upon the upgrade to Gajim v1.*, I've been greeted with quite unreadable chat windows.

The issues stem from overriding the style of standard controls.

• elements blend into each other
• no visual indication of what's a button, what's not
• no indication of which of the buttons are unclickable

Please notice how positioning the cursor over the button doesn't cause it to reveal its function as a button.

The gajim.css file overrides several builtin styles for a couple of standard interface elements: windows, buttons, and scroll bars (.chatcontrol-actionbar-button, .scrollbar-style, #MessageWindow, #RosterWindow paned. I find it confusing, as this causes Gajim's chat window to lose consistency with the rest of the destop environment, and even with other windows of its own (like account settings).

I tried to see whether there was a good reason to override user's preferences - perhaps leaving the defaults causes hard to predict bugs. As it turns out, with my theme, the roster window is at least as useable as before.

Because I think it's an advantage to stay consistent across the whole system, I decided to submit this change as a merge request.