Fix window close, icon toggle logic

"Toggle" should mean "show or hide all windows", not "flip the visibility state
of windows". Case: one receiver window is open. When a new receiver is
connected, I also expect it to be open. Instead it is hidden, so I click the
icon. Now the new receiver window is shown, but the previous one is hidden.
Huh?! Indeed, let's fix that.
Fix window positioning of main windows

Previously, the intent was likely to position the receiver window near the
status icon. It did so by calling move followed by present. According to the
Gtk documentation, move() may fail if the window is hidden before.

Therefore present the window *after* determining the position, but *before*
moving it. (presenting the window before getting the position gives a Gtk
warning and has unpredictable behavior wrt. the window position).

As window positioning is now enabled, add additional logic to prevent overlap
of windows: position the first window near the status icon and others on the
left. This is not idea, e.g. when the status icon is on the left side of the
screen, but the idea of positioning windows near to each other is broken anyway.
Merge pull request #40 from Lekensteyn/fixes

Fix window close, fix positioning of new windows