Restore `-[SLTextField setText:]`'s use of `-[SLKeyboard typeString:]` in all cases.

This effectively reverts commit dc491fc72218a4d1c849f27a5482ec9adac620c9.
The UIAutomation bug described by that commit could not be reproduced:
`UIAKeyboard.typeString` appears to change the keyplane as necessary
to type special characters, across all device type and SDK combinations.

Pending further bug reports, using `-typeString:` is to be preferred
because it is simpler, because it most accurately simulates user interaction,
and because if there was a bug discouraging the use of `-typeString:`, a workaround
would have to be implemented not just in `-[SLTextField setText:]` but also
in `-setText:` as implemented by `SLWebTextField`, `SLTextView`, and `SLWebTextView`.

A test case has been added to verify that `-[SLKeyboard typeString:]` changes
the keyplane as necessary to type special characters.
Create a specific test case for `-[SLTextField setText:]` clearing the current text.
Make logic around text fields and views becoming first responder consistent.
All text fields and views now clear their current text before setting new text.
Clarify the use of `SLWebTextView` to represent HTML text areas.

Despite UIAutomation representing such DOM elements by instances of `UIATextField`.
Merge pull request #58 from inkling/jeff/revert_SLTextField_setText_workaround

Text field and view fixes (followup to #52).