@wvmarle - There are many apps that can be easily substituted with Google :-D. But they still exist and have many users. An app gives a solid offline database that can be accessed anytime. You are on the train and want to choose boards and sensors for your next project, well then you have it there.
I do keep some frequently used datasheets offline on my computer, that's some 180 MB for just 71 of them. That includes a number of ATMEL datasheets for various processors, a number of sensors that I happen to use in different projects, some MOSFETs, etc. Your database will get really big, fast.
Of course that's not complete, but I've never seen that as an issue - just tethering my laptop to my phone and download as needed. For any useful database, without giving the users the feeling that "most parts are missing!" you need hundreds more datasheets.
A phone is anyway quite hopeless for studying datasheets. Way too small a screen, way too hard to navigate.
I have tons of sensors, boards and random parts at home. When I come to do a project I don't know if I have a DHT11 sensor without looking, so I wanted an app that would help me keep stock. An app with a built-in library of parts.
Now all that's left to do is tediously keep your stock up to date! That includes updating it every time you take a part and use it in a project, and when your project is done and you recover the parts make sure they're all added back in.
I've sorted all in a couple dozen compartment boxes, roughly categorised. That's enough to answer the question "do I have this part" within a few minutes, if I think I may have it. Otherwise I'll just order another 5-10 of it. Most parts and sensors are cheap, unused ones just get added to the parts box.
work as a Bluetooth controller for your Arduino
How do you see this? Considering the hundreds of different controls imaginable, how will you make sure this is not hopelessly limited as well? I don't think any such program exists - definitely not for the generic case. There's a reason for that: there is no such thing as a generic control for an Arduino! Most that I know of that do Bluetooth control will start with a Bluetooth terminal, and then create a simple app with App Inventor or so, that's tailored to their specific use case.
I think keeping it up to date is not so hard. The language Arduino uses rarely changes and there is not a new board every week, so personally I believe it will be minor updates.
The reference manual for C++ is no doubt HUGE, especially if you want explanations and examples on the various commands. Add to that the manual of the Arduino specific functions (is that even available as download? I only ever read this part online).