Let site plugins use the ionViewCanLeave function

This issue only needs to be tested in 1 device. Install the plugin attached to this issue, inside the "local" folder. In the app, login in that site and go to the More tab. Check that you see an option called "Can leave test" or "[ [pluginname] ]" (I did something wrong when creating the lang of the plugin and I didn't want to lose time on that, I'm not sure if it was fixed with the latest changes). Click the option. Check that you see a new page saying "This is a test!". Click the back button. Check that you see a confirm message. Click "Cancel". Check that you stay in the page. Now click back again and now confim. Check that you're sent to the previous page.