Viewer suggestions

[Dreamer]

Few suggestions for viewer...

- When using option "Use big icons", use big icons also for image toolbar.
- "Show/hide main menu" option, button and keyboard shortcut to toggle and for menu button, as in main FC window.
- "Rename" file in viewer, with F2 shortcut.
- Auto-hide toolbars in fullscreen mode.
- Default shortcut for "Fit to window" is Num*, in XnView it's Num/ and Num* is for "Original size, I suggest to use the same shortcuts as in XnView.