[3.6] bpo-30495: IDLE: improve textview with docstrings, PEP8 names, more tests....

[3.6] bpo-30495: IDLE: improve textview with docstrings, PEP8 names, more tests. (GH-2283) (#2496)

Split TextViewer class into ViewWindow, ViewFrame, and TextFrame classes so that instances
of the latter two can be placed with other widgets within a multiframe window.
Patch by Cheryl Sabella.
(cherry picked from commit 42bc8be)