bug(developer): Key processing with text selection problems

[ermshiperete]

The test cases for the Text Selection and Key Processing spec shows some limitations in Keyman Developer (Debugger Mode).

See here for the test cases and the google doc for the test results.

Failed tests:

1. (T04) ^⁞X⁞ in doc. Typing A.
   Expected result: Â⁞
   Actual result: ^A⁞
2. (T05) ^⁞X the quick brown fox⁞ in doc. Typing A.
   Expected result: Â⁞
   Actual result: ^A⁞
3. (T15) Type X, a followed by [K_BKQUOTE] and b. Select b (Xa⁞b⁞).
   Press BKSP twice.
   Expected result: X
   Actual result: Xfail2
4. (T16) Type X, a followed by [K_BKQUOTE] and b. Select b (Xa⁞b⁞).
   Press BKSP then o.
   Expected result: Xao
   Actual result: Xaok3

[mcdurdin]

For the debugger, one difference of note is that markers are persisted across cursor movement, unlike in ordinary text input scenarios; this may not affect the tests reported here but is something to be aware of. Additionally, markers are visible in the character view pane in the debugger, which can help.