Improvements to how the VoIP panel is aligned on the screen

I'm not satisfied with how aligning the VoIP panel on the user's screen works. Right now, voice_showpanel is a boolean that controls if whether or not it should appear at all. If voice_panelx or voice_panely are negative, then the VoIP panel is aligned to the right or bottom of the screen respectively. This means that there's no way to align it to the right/bottom without there always being some offset, which might not be desirable.

This MR changes voice_showpanel to an integer CVar, in which users choose the corner of the screen they'd like the panel to be aligned to: top-left, bottom-left, top-right, or bottom-right. Likewise, voice_panelx and voice_panely no longer accept negative values.