git-gui: Fix window manager problems on ion3

git-gui: Fix window manager problems on ion3

cehteh on #git noticed that secondary windows such as console
windows from push/fetch/merge or the blame browser failed on ion
when we tried to open them a second time.

The issue turned out to be the fact that on ion [winfo ismapped .]
returns false if . is not visible right now because it has been
obscured by another window in the same panel.  So we need to keep
track of whether or not the root window has been displayed for this
application, and once it has been we cannot ever assume that ismapped
is going to return true.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>