Fix for #446 and #453

[in3otd]

lot of small stuff here,

• implemented changing the Qucs Home without restarting: it's a bit hackish but seems to work. Should have divided the "close all open files" and "prepare a new empty schematic and reload the projects tree" into two separate functions and maybe added a QMessageBox to inform the user.
• changed the non-native file dialogs (there were several).
• corrected handling the "Task" and "Comment" colors which has never been properly saved/loaded. They seem actually unused, though 😕
• fixed the crash when changing background color of a Text Document, Crash when changing background color of text document #453
• corrected the behaviour of "Add Path With Subfolders", see Change in Qucs Home requires restart. #446
• removed the remaining inherits() used to determine if a widget was a TextDoc or a Schematic, used a static function defined in QucsApp instead. Confirmed that also the simulation of a text document was crashing due to the wrong check.
• removed the deprecated Qt3 compatibility functions in in qucssettingsdialog.cpp; could have been done a bit more cleanly but lacked time...

Tried to check everything, seems to work here, please double check there also, thanks.

[in3otd]

? AppVeyor fails with

./.libs/lt-admsXml.c:315:19: error: '_P_WAIT' undeclared (first use in this function)
rval = _spawnv (_P_WAIT, lt_argv_zero, (const char * const *) newargz);
^

also #458 .

EDIT: added 0.0.19 label, since at least 9a1eada should go in to fix the GUI crashes

[guitorri]

Related to unused QSettings, this is now #460

[guitorri]

Thank you @in3otd.
The background color now works for the schematic and text documents. Bug or feature?
Perhaps we should allow two colors? Or just white for the text document...

Ignore AppVeyor for the moment.... they updated something that broke something else I don't yet understand...

[in3otd]

The background color now works for the schematic and text documents. Bug or feature?

well, since the original code tried to set the text documents background color to QucsSettings.BGColor (but didn't work), I made it actually work as intended. I think we may call it a feature 😁

But we can certainly have two different background colors for the two document types, in 0.0.20 or later...

[guitorri]

Fair enough.
I build it locally on windows. We can skip the AppVeyor issue for the moment.
Thank you!