[FEATURE] [DB manager] allow to store SQL queries for quick access

Hi !

Here's a little mod to the DB Manager plugin, allowing to store SQL queries in the project file for reuse.

I'm not sure if this is allowed during feature freeze (since it's not the core). If not, no problem to keep this for 2.1...

https://www.dropbox.com/s/yiy9v2nvspmxl6e/screenshot.png