Fix parse of inner layout

[304NotModified]

Parse of inner layout was not always correct working. Escaping of colon and bracket was only with hack (${literal:text=\:}) possible. This is now fixed. It does still need an escape with \. The work around is also still working.

fixes #444 and #914

Working examples:

• ${when:when=1 == 1:Inner=Test\: Hello}
• ${when:when=1 == 1:Inner=Test\\Hello}
• ${when:when=1 == 1:Inner=Test\Hello}
• ${when:when=1 == 1:Inner=Test{Hello\}}

We need to escape : and } in an inner layout because:

• : because it's a value separator.
• } because it's the end of the layout

note: no need to escape \.

Fixes #914 confirmed with unit tests ("ComplexWhenWithHashTest" fails with master)

[codecov-io]

Current coverage is 70.48%

Merging #975 into master will not affect coverage as of fde4423

@@            master    #975   diff @@
======================================
  Files          262     262       
  Stmts        14770   14786    +16
  Branches      1610    1613     +3
  Methods          0       0       
======================================
+ Hit          10410   10422    +12
- Partial        398     399     +1
- Missed        3962    3965     +3

Review entire Coverage Diff as of fde4423

---

Uncovered Suggestions

1. +0.09% via ...oredConsoleTarget.cs#232...244
2. +0.08% via ...c/NLog/LogFactory.cs#180...191
3. +0.07% via ...anceCounterTarget.cs#308...318
4. See 7 more...

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