Add PLCOpenEditor icon to plcopen_icons.svg

add 256x256 size to the icon

Fix rounding of thousandths of microseconds in duration editor

Add checking value in microseconds field in duration editor dialog

Catch 'clipboard already open' errors in editors

Apparently following commits have not fixed the problem completely,
because it still happens (although not so often).

changeset: 2019:b8f795bdfe9f
user: Andrey Skvortsov <andrej.skvortzov@gmail.com>
date: Fri May 25 17:40:29 2018 +0300
summary: fix hangs on mouse selection in case if wxPython uses wxWidgets with GTK3+ support

changeset: 2018:5ce6d08ff2c7
user: Andrey Skvortsov <andrej.skvortzov@gmail.com>
date: Fri May 25 17:23:15 2018 +0300
summary: make clipboard open minimal time as wxPython documentation recommends