Import fix from upstream Scintilla to fix crash on drag and drop

This imports Scintilla commit 06f36a3217229ec2c2a360e370ae1b07597ce7b6
from HG, "Fix crashes with SelectionText instances that did not have \0
terminators."
Import fix from upstream Scintilla to fix crash upon DnD with GTK3

This imports Scintilla commit e488315f2d30e1795f0899779bac948da41b4eef
from HG, "Bug: [#1481]. Incorrect event copying on GTK+ 3.x."