Kellyroach/fix warnings

[kellyroach]

This pull request fixes assorted Xcode 9.4.1 compiler warnings.

• IPHONEOS_DEPLOYMENT_TARGET = 8.0
  This change extends support down to iOS 8.0 from iOS 9.2 and
  avoids yellow warnings regarding 8.0 versus 9.2 linkage to iosMath .
• Xcode 9.4.1 Update to recommended settings
• Fix 'Block implicitly retains self' yellow warning
• Fix 'Dead Store' Analyze issue
• Fix 'nil returned from a method' Analyze issue
• Fix another 'nil returned from a method' Analyze issue
• Fix 'Could not load the keyboard-marine-pressed image' runtime error
  Capitalization issue in MTKeyboardTab3.xib
  Use Keyboard-marine-pressed with uppercase K .

We filtered iosMath Analyze issues by modifying Podfile at start of our
commit trail, then restoring original Podfile before we branched at end
of our commit trail. Our recently approved and merged P.R. #103 on
the iosMath repo addresses the iosMath Analyze issues not addressed
by the current P.R. here.

We tested MathEditor on our branch just before and just after restoring
original Podfile . The app functions OK in both tests. The difference is
we do see remaining iosMath Analyze issues after restoring original Podfile
because our P.R. #103 changes to the iosMath repo haven't been issued
a release on COCOAPODS.ORG yet.

[kellyroach]

TEST: before restoring original Podfile

• 2 Analyze targets --> "No issues" PASSED
• Build and Run app on "iPhone X" simulator
  Entered x/y+x^y >= 3.14159 and it looks OK
  PASSED

TEST: after restoring original Podfile (current state of our P.R. branch)

• Analyze --> "3 warnings, 25 analyzer results"
  which all seem to be from "Analyze target iosMath" as expected.
  There is a good chance most or all of these will go away after next
  release of iosMath to COCOAPODS.ORG, assuming it contains our
  P.R. #103 on the iosMath repo changes. Without our changes in
  current P.R. we will get "4 warnings, 28 analyzer results".
• Build and Run app on "iPhone X" simulator
  Entered x/y+x^y >= 3.14159 and it looks OK
  PASSED

[kellyroach]

Product > Test before restoring original Podfile

Test Suite 'MTDisplayEditingTest' passed at 2018-07-12 21:06:33.929.
	 Executed 5 tests, with 0 failures (0 unexpected) in 0.079 (0.082) seconds
Test Suite 'MathEditor_Tests.xctest' passed at 2018-07-12 21:06:33.930.
	 Executed 5 tests, with 0 failures (0 unexpected) in 0.079 (0.083) seconds
Test Suite 'All tests' passed at 2018-07-12 21:06:33.930.
	 Executed 5 tests, with 0 failures (0 unexpected) in 0.079 (0.085) seconds

Product > Test after restoring original Podfile

Test Suite 'MTDisplayEditingTest' passed at 2018-07-12 21:09:18.971.
	 Executed 5 tests, with 0 failures (0 unexpected) in 0.057 (0.059) seconds
Test Suite 'MathEditor_Tests.xctest' passed at 2018-07-12 21:09:18.972.
	 Executed 5 tests, with 0 failures (0 unexpected) in 0.057 (0.060) seconds
Test Suite 'All tests' passed at 2018-07-12 21:09:18.973.
	 Executed 5 tests, with 0 failures (0 unexpected) in 0.057 (0.062) seconds

[kellyroach]

XCTOOL TEST ON OUR LOCALHOST

2015s-MacBook-Pro:MathEditor kellyroach$ xctool -workspace MathEditor.xcworkspace -scheme iosMathEditor-Example run-tests -test-sdk iphonesimulator
[Info] Loading settings for scheme 'iosMathEditor-Example' ... (733 ms)

=== RUN-TESTS ===

  [Info] Collecting info for testables... (942 ms)
  run-test MathEditor_Tests.xctest (iphonesimulator11.4, iPhone 5s, logic-test)
    ✓ -[MTDisplayEditingTest testClosestPointExponent] (16 ms)
    ✓ -[MTDisplayEditingTest testClosestPointFraction] (0 ms)
    ✓ -[MTDisplayEditingTest testClosestPointFractionPlusRegular] (0 ms)
    ✓ -[MTDisplayEditingTest testClosestPointRegular] (0 ms)
    ✓ -[MTDisplayEditingTest testClosestPointRegularPlusFraction] (0 ms)
    5 passed, 0 failed, 0 errored, 5 total (19 ms)


** RUN-TESTS SUCCEEDED: 5 passed, 0 failed, 0 errored, 5 total **