(MainWindow): various tweaks.

- set ImageData default DPI to desktop resolution.
- display menubar on ALT keypress when it's hidden.
- tweaked filename lookup on keypress logic (search should start from
the current position, not from beginning).

MainWindow.xaml

@@ -6,6 +6,7 @@
         xmlns:p="clr-namespace:GARbro.GUI.Properties"
         Title="GARbro" MinHeight="250" ResizeMode="CanResizeWithGrip"
         Loaded="WindowLoaded"
+        KeyDown="WindowKeyDown"
         Top="{Binding Source={x:Static p:Settings.Default}, Path=winTop, Mode=TwoWay}"
         Left="{Binding Source={x:Static p:Settings.Default}, Path=winLeft, Mode=TwoWay}"
         Height="{Binding Source={x:Static p:Settings.Default}, Path=winHeight, Mode=TwoWay}"