Add an option to move search box above application's plugin

[flexiondotorg]

Originally reported by: Dionisio E Alonso (Bitbucket: baco, GitHub: baco)

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The search-box is fixed to the bottom part of the menu, just underneath the application's plugin. That view is fine when the menu is placed in a bottom panel, as it is the default in Linux Mint.

When using mate-menu in a top panel it becomes more natural to find the search box near the menu button itself, placing the search-box on top of the application's plugin would achieve that. Adding an option to move the search-box on top of the application's plugin would be enough.

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• Bitbucket: https://bitbucket.org/ubuntu-mate/mate-menu/issue/11