Improve UI for changing the "Max. memory" profile setting

[herzbube]

Currently it is very easy for the user to change the "Max. memory" profile setting to a value that is too high. The UI should be improved so that the user is amply warned when she changes that setting.

• Create a dedicated screen for changing the setting
• When she changes the setting (possibly only when she increases the value), the user must confirm that she knows what she is doing
• Add information about how much memory all known iOS device have
• Add a tappable link that takes the user to the Wikipedia page where all iOS devices with their capabilities are listed

[herzbube]

The last two points have been scrapped, instead the view now limits the upper limit of the setting to a fraction of the amount of physical memory that the device has. The fraction increases with the amount of physical memory. Thresholds have been set like this:

• <= 256 MB: Upper limit is 1/4 of physical memory. This is probably a bit too aggressive: On an iPhone 3GS setting "Max. memory" to the upper limit of 64 MB crashes the app if the user also uses zoom. Still, I wanted to give the user the freedom to not use zoom in favor of giving more memory to the GTP engine.
• 257-512 MB: Upper limit is 1/2 of physical memory. This should be fine, even if the user uses zoom.

• = 512 MB: Upper limit is 2/3 of physical memory. This is probably a bit too conservative, but better be safe than sorry, and it's still an improvement over the old hard-coded upper limit of 512 MB.