Convert internal, static colors into color assets. (#1386)

* Add Assets.xcassets to MessageKitAssets.bundle

* Replace incomingGray, outgoingGreen, inputBarGray

* Replace playButtonLightGray, with UIVisualEffectView to mimic iMessage (Fixes #1321 , Fixes #1335)

* Fixes an issue where the triangle in the play button is not centered

* remove sendButtonBlue, add incoming/outgoingAudioMessageTint

Matches audio tint more closely to iMessage by tinting white on outgoing and tinting blue on incoming

* Remove backgroundColor, add collectionViewBackground

* remove unused labelColor

* Remove unused placeholderTextColor

* Remove grayColor

* remove darkTextColor, add typingIndicatorDot

Matches dot color more closely to iMessage

* Remove unused lightGrayColor

* Adds high contrast appearances to the color assets

* Add MessageKitError string for case when color asset could not be loaded

Also removes redundant internal access level modifier on MessageKitError (internal is default), and changes MessageKitError from enum -> struct as its members were not enum cases

* Fix swiftlint issues

* Add label color asset and use in ContactMessageCell

* Fix tests

MessageKit.podspec

@@ -14,7 +14,8 @@ Pod::Spec.new do |s|
    s.swift_version = '5.0'
 
    s.ios.deployment_target = '11.0'
-   s.ios.resource_bundle = { 'MessageKitAssets' => 'Assets/MessageKitAssets.bundle/Images' }
+   s.ios.resource_bundle = { 'MessageKitAssets' => ['Assets/MessageKitAssets.bundle/Images'] }
+   s.ios.resources = ['Assets/Assets.xcassets']
 
    s.dependency 'InputBarAccessoryView', '~> 4.3.0'