Add indent/dedent to python editor in scripter

Add indent/dedent functionality to editor that ships with scripter
script.
Previously tab key added 4 spaces whereever the cursor was. With this
commit tab indents selected lines by 4 characters and shift-tab dedents
by a the same amount. Empty lines are not indented and dedenting stops
once code has reached left hand margin.

4 chars is hard coded but could be a setting later. Chose 4 chars
for style consistency. Default editor window should be widened to 79
chars.

Differential Revision: https://phabricator.kde.org/D9419