Crash when changing background color of text document

[in3otd]

When changing a text documents background color (using the Application Settings dialog) Qucs crashes.
To reproduce, open Qucs, create a new text document (File->New Text) and change the Application background color.

The problem comes from here, as now TextDoc is derived from QPlainTextEdit.

I'll put a fix together with the other stuff for #446 but feel free to push a separate hotfix if you would like to be sure that it will be in 0.0.19 😁

EDIT:
a grep -r "inherits" --include \*.cpp . shows also some other suspect things:

./qucs/dialogs/simmessage.cpp:  if(DocWidget->inherits("QTextEdit"))
./qucs/dialogs/simmessage.cpp:  if(!DocWidget->inherits("QTextEdit")) {
./qucs/dialogs/simmessage.cpp:  if(DocWidget->inherits("QTextEdit")) {

[guitorri]

Confirmed on OSX. If you find a fix please inform.

[in3otd]

...well, fix is just to use inherits("QPlainTextEdit") instead of inherits("QTextEdit"), as said it will be in the PR for #446, not sure I can finish it today.

Could you/someone please take a look if there is a similar bug related to the inherits("QTextEdit") in simmessage.cpp ? At a first glance it's about simulating VHDL/Verilog text files in Qucs, but I never did this, so I cannot check if it's a real issue.

BTW, there is a function QucsApp::isTextDocument which is extensively used in qucs.cpp for this check, will be nice to reuse it around, so if we change the TextDoc inheritance another time just changing this function will be enough.
What will be the best way to do this? Declare it static? Or outside of a class, in misc.cpp ?