Explore mode

Hello,

I've added a draft implementation of this "Explore mode" feature allowing to search venues based on some filters. It's still not perfect but seems to be usable.
Hope the changes will be accepted, however I think something may be improved based on code review of my changes, since I'm not an experienced QML/JS developer, sorry.

Just a couple of points that might need some care:
1. Explore mode activation, sometimes application crashes when you click on the map to go to explore mode page before map is loaded. Also this way to open explore mode is not really obvious for someone who did not have a look at the code. I guess there should be something like an arrows listbox icon in the page header or whatever.
2. Adding veneues to display search results, for some reason ListView.onAdd and ListView.onRemove is called only for the first 7 entries, so only these entries have corresponding icons on map. I have an ugly hack that adds an icon when the venue is selected from list, but I think there should be a better way to fix it. Maybe I used QML api incorrectly...
3. Venue icon and text, text is sometimes under the icon which is bad, and it all looks bad, maybe it worth to use an icon form foursquare website (text is a part of icon, icon colors inverted when venue is selected, specials have their own color).
4. I have not made any changes to localization

Thanks for the awesome project :)

Regards,
Constantine