+Open menu item with blank handler
+OpenFileDialog to the form
Setting file filter to image files
Consolidating the 'Open' terminology
Showing open dialog when using the menu item
Setting Ctrl+O as shortcut for Open
Setting the window background to the selected file
+Close menu item and separator (and blank handler)
+HasFileOpen flag

- Used to denote when a file is opened
- Used by the new UpdateUI() method to toggle enabled-ness of controls
- UpdateUI() is called on load, and will be called on major events
- UpdateUI() is called on HasFileOpen flag set
Close menu item option now disposes and nulls the background image

Also resets the flag.