Add memory pressure indication in devbuilds

[pavelz]

Background

ABB is still performing slightly worse than other ​UIWebView based browsers. This results in ABB being more prone to crashing due to memory pressure. In order to get a better understanding of which situations cause higher memory pressure, we'd like to introduce an indicator, which visualises the memory consumption in real time.
As this function is for testing purpose only, we will exclude this feature in App Store versions and only include it in development builds.

What to change

Only show the functions/elements below in development builds.
Use the ​standard iOS control style for any control/element described below.

1. Settings

1.1 Settings overview
Add a new settings item to the "General" group as shown in this screenshot.

Settings item text: Devbuild Settings
Click event: Display the new "Devbuild Settings" screen as described under 1.2

1.2 Devbuild Settings
Add a new settings group called "Memory Pressure" to the new "Devbuild Settings" screen as shown in this screenshot.

Settings group text: Memory Pressure

Add a new switch called "Show Memory Pressure Indication" to the new "Memory Pressure" group in the "Devbuild Settings" screen as shown in this screenshot

Switch control text: Show Memory Pressure Indication
Default state: Unchecked
Checked event: Display the memory pressure indication while browsing as described under 2.
Unchecked event: Do not display the memory pressure indication while browsing as described under 2.
Save state: Changing the checked state will be saved and persists upon restart

2. Memory Pressure Indicator

If the "Show Memory Pressure Indication" switch (as described under 1.2) is checked, show the memory pressure indicator within the settings bar as shown in this screenshot with the following anntributes:

Text: Memory pressure: <n>MB/<m>MB where <n> is replaced by the currently used memory and <m> is replaced by the maximum memory. Both figures in MB.
Color: Standard grey
Position: bottom, center

Remark for testers

You can use ​this stress test to drive the memory usage of ABB into Gigabytes.

[pavelz]

​https://www.pivotaltracker.com/story/show/121206803

[scheer]

• The new settings item 'Devbuild Settings' has been added to the 'General' group.
• A new setting group called "Memory Pressure" has been added to the "Devbuild Settings" screen.
• The 'Show Memory Pressure Indication' switch is now added to the new 'Memory Pressure' group.
• Default state: The option remains unchecked.
• Checked event: The memory pressure is shown while browsing as described under 2.
• Unchecked event: No memory pressure indicator is shown while browsing.
• Save state: Changing the checked state is saved and persists upon restart.

ABB 1.5.0 (1066)
iPhone 6s Plus
iOS 9.3.1