Implement support for loading/saving mappings in iOS (Issue 131)

[adamjansch]

For iOS, implemented loading of bundled mappings, and loading/saving of user mappings. Original issue at #131.

This also fixes a crash that occurs when accessing MIKMIDIMappingManager.shared() when there is a .midimap file present in the bundle (aka: a bundled mapping).

[armadsen]

Thanks so much for this! I will review it and merge it and/or give feedback as soon as I can.

[armadsen]

@aj-bmdi: Do you mind sending a .midimap file that caused a crash before your fix was implemented?

[adamjansch]

@armadsen Sure, this is what I tested with (and what was causing the crash):

<?xml version="1.0" encoding="UTF-8"?>
<Mapping ControllerName="Network" MappingName="Network">
    <MappingItems>
	    <MappingItem InteractionType="Tap" Flipped="false">
		    <ResponderIdentifier>button-0</ResponderIdentifier>
		    <CommandIdentifier>ButtonDown</CommandIdentifier>
		    <Channel>0</Channel>
		    <CommandType>191</CommandType>
		    <ControlNumber>19</ControlNumber>
	    </MappingItem>
	    <MappingItem InteractionType="Key" Flipped="false">
		    <ResponderIdentifier>button-6</ResponderIdentifier>
		    <CommandIdentifier>ButtonDown</CommandIdentifier>
		    <Channel>0</Channel>
		    <CommandType>191</CommandType>
		    <ControlNumber>22</ControlNumber>
	    </MappingItem>
	    <MappingItem InteractionType="KnobSlider" Flipped="true">
		    <ResponderIdentifier>slider-0</ResponderIdentifier>
		    <CommandIdentifier>ValueChanged</CommandIdentifier>
		    <Channel>0</Channel>
		    <CommandType>191</CommandType>
		    <ControlNumber>126</ControlNumber>
	    </MappingItem>
	    <MappingItem InteractionType="KnobSlider" Flipped="false">
		    <ResponderIdentifier>slider-6</ResponderIdentifier>
		    <CommandIdentifier>ValueChanged</CommandIdentifier>
		    <Channel>0</Channel>
		    <CommandType>191</CommandType>
		    <ControlNumber>6</ControlNumber>
	    </MappingItem>
    </MappingItems>
</Mapping>

The crash was occurring when assigning the control number to the new mapping item, as the latter requires an unsigned int but the former was a coming through as a string and NSString has no method to convert to that type directly.

[armadsen]

Looks great, thanks!